Method of in-wafer testing of monolithic photonic integrated circuits (PICs) formed in a semiconductor wafer

ABSTRACT

A method of in-wafer testing is provided for a monolithic photonic integrated circuit (PIC) formed in a semiconductor wafer where each such in-wafer circuit comprises two or more integrated electro-optic components, one of each in tandem forming a signal channel in the circuit. The method includes the provision of a first integrated photodetector at a rear end of each signal channel and a second integrated photodetector at forward end of each signal channel. Then, the testing is accomplished, first, by sequentially operating a first of a selected channel electro-optic component in a selected circuit to monitor light output from a channel via its first corresponding channel photodetector and adjusting its operating characteristics by detecting that channel electro-optic component output via its second corresponding channel photodetector to provide first calibration data. Second, by sequentially operating a second of a selected channel electro-optic component in the selected circuit to monitor signal output from the second selected channel electro-optic component via its second corresponding channel photodetector and adjusting its operating characteristics by detecting that channel electro-optic component output via its second corresponding channel photodetector to provide second calibration data. The first and second calibration data for each circuit channel for the selected circuit are then stored for future reference.

REFERENCE TO RELATED APPLICATION

This application is division of patent application, Ser. No. 10/317,935,filed Dec. 11, 2002, which is a continuation of patent application, Ser.No. 10/267,331, filed Oct. 8, 2002, also published on May 22, 2003 asPublication No. US2003/0095737A1, which claims priority to the followingprovisional applications: Ser. No. 60/60/328,207, filed Oct. 9, 2001;Ser. No. 60/328,332, filed Oct. 9, 2001; Ser. No. 60/370,345, filed Apr.5, 2002; Ser. No. 60/378,010, filed May 10, 2002; Ser. No. 60/392,494,filed Jun. 28, 2002; and Ser. No. 60/367,595, filed Mar. 25, 2002, allof which applications are also assigned to a common assignee and areincorporated herein by their reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates generally to optical telecommunication systemsand optical transport networks employed in such systems deployingphotonic integrated circuits (PICs) for wavelength division multiplexed(WDM) or dense wavelength division multiplexed (DWDM) optical networksand more particularly to testing of PICs employed in such systems whilethe circuits are still part of a semiconductor wafer.

2. Description of the Related Art

If used throughout this description and the drawings, the followingshort terms have the following meanings unless otherwise stated:

1R—Re-amplification of the information signal.

2R—Optical signal regeneration that includes signal reshaping as well assignal regeneration or re-amplification.

3R—Optical signal regeneration that includes signal retiming as well assignal reshaping as well as re-amplification.

4R—Any electronic reconditioning to correct for transmission impairmentsother than 3R processing, such as, but not limited to, FEC encoding,decoding and re-encoding.

A/D—Add/Drop.

APD—Avalanche Photodiode.

AWG—Arrayed Waveguide Grating.

BER—Bit Error Rate.

CD—Chromatic Dispersion.

CDWM—Cascaded Dielectric wavelength Multiplexer (Demultiplexer).

CoC—Chip on Carrier.

DBR—Distributed Bragg Reflector laser.

EDFAs—Erbium Doped Fiber Amplifiers.

DAWN—Digitally Amplified Wavelength Network.

DCF—Dispersion Compensating Fiber.

DEMUX—Demultiplexer.

DFB—Distributed Feedback laser.

DLM—Digital Line Modulator.

DON—Digital Optical Network as defined and used in this application.

EA—Electro-Absorption.

EAM—Electro-Absorption Modulator.

EDFA—Erbium Doped Fiber Amplifier.

EML—Electro-absorption Modulator/Laser.

EO—Electrical to Optical signal conversion (from the electrical domaininto the optical domain).

FEC—Forward Error Correction.

GVD—Group Velocity Dispersion comprising CD and/or PMD.

ITU—International Telecommunication Union.

MMI—Multimode Interference combiner.

MPD—Monitoring Photodiode.

MZM—Mach-Zehnder Modulator.

MUX—Multiplexer.

NE—Network Element.

NF—Noise Figure: The ratio of input OSNR to output OSNR.

OADM—Optical Add Drop Multiplexer.

OE—Optical to Electrical signal conversion (from the optical domain intothe electrical domain).

OEO—Optical to Electrical to Optical signal conversion (from the opticaldomain into the electrical domain With electrical signal regenerationand then converted back into optical domain) and also sometimes referredto as SONET regenerators.

OEO-REGEN—OEO signal REGEN using opto-electronic regeneration.

OO—Optical-Optical for signal re-amplification due to attenuation. EDFAsdo this in current WDM systems.

OOO—Optical to Optical to Optical signal conversion (from the opticaldomain and remaining in the optical domain with optical signalregeneration and then forwarded in optical domain).

OOO-REGEN—OOO signal REGEN using all-optical regeneration.

OSNR—Optical Signal to Noise Ratio.

PIC—Photonic Integrated Circuit.

PIN—p-i-n semiconductor photodiode.

PMD—Polarization Mode Dispersion.

REGEN—digital optical signal regeneration, also referred to asre-mapping, is signal restoration, accomplished electronically oroptically or a combination of both, which is required due to bothoptical signal degradation or distortion primarily occurring duringoptical signal propagation caused by the nature and quality of thesignal itself or due to optical impairments incurred on the transportmedium.

Rx—Receiver, here in reference to optical channel receivers.

RxPIC—Receiver Photonic Integrated Circuit.

SDH—Synchronous Digital Hierarchy.

SDM—Space Division Multiplexing.

Signal regeneration (regenerating)—Also, rejuvenation. This may entail1R, 2R, 3R or 4R and in a broader sense signal A/D multiplexing,switching, routing, grooming, wavelength conversion as discussed, forexample, in the book entitled, “Optical Networks” by Rajiv Ramaswami andKumar N. Sivarajan, Second Edition, Morgan Kaufmann Publishers, 2002.

SMF—Single Mode Fiber.

SML—Semiconductor Modulator/Laser.

SOA—Semiconductor Optical Amplifier.

SONET—Synchronous Optical Network.

SSC—Spot Size Converter, sometimes referred to as a mode adapter.

TDM—Time Division Multiplexing.

TEC—Thermo Electric Cooler.

TRxPIC—Monolithic Transceiver Photonic Integrated Circuit.

Tx—Transmitter, here in reference to optical channel transmitters.

TxPIC—Transmitter Photonic Integrated Circuit.

VOA—Variable Optical Attenuator.

WDM—Wavelength Division Multiplexing. As used herein, WDM includes DenseWavelength Division Multiplexing (DWDM).

DWDM optical networks are deployed for transporting data in longhaul-networks, metropolitan area networks, and other opticalcommunication applications. In a DWDM system, a plurality of differentlight wavelengths, representing signal channels, are transported orpropagated along fiber links or along one more optical fibers comprisingan optical span. In a conventional DWDM system, an optical transmitteris an electrical-to-optical (EO) conversion apparatus for generating anintegral number of optical channels λ₁, λ₂, λ_(N), where each channelhas a different center or peak wavelength. DWDM optical networkscommonly have optical transmitter modules that deploy eight or moreoptical channels, with some DWDM optical networks employing 30, 40, 80or more signal channels. The optical transmitter module generallycomprises a plurality of discrete optical devices, such as a discretegroup or array of DFB or DBR laser sources of different wavelengths, aplurality of discrete modulators, such as, Mach-Zehnder modulators(MZMs) or electro-absorption modulators (EAMs), and an optical combiner,such as a star coupler, a multi-mode interference (MMI) combiner, anEchelle grating or an arrayed waveguide grating (AWG). All of theseoptical components are optically coupled to one another as an array ofoptical signal paths coupled to the input of an optical combiner using amultitude of single mode fibers (SMFs), each aligned and opticallycoupled between discrete optical devices. A semiconductormodulator/laser (SML) may be integrated on a single chip, which in thecase of an electro-absorption modulator/laser (EML) is, of course, an EAmodulator. The modulator, whether an EAM or a MZM, modulates the cwoutput of the laser source with a digital data signal to provide achannel signal which is different in wavelength from each of the otherchannel signals of other EMLs in the transmitter module. While eachsignal channel has a center wavelength (e.g., 1.48 μm, 1.52 μm, 1.55 μm,etc.), each optical channel is typically assigned a minimum channelspacing or bandwidth to avoid crosstalk with other optical channels.Currently, channel spacings are greater than 50 GHz, with 50 GHz and 100GHz being common channel spacings.

An optical fiber span in an optical transport network may providecoupling between an optical transmitter terminal and an optical receiverterminal. The terminal traditionally is a transceiver capable ofgenerating channel signals as well as receiving channel signals. Theoptical medium may include one or more optical fiber links forming anoptical span with one or more intermediate optical nodes. The opticalreceiver receives the optical channel signals and converts the channelsignals into electrical signals employing an optical-to-electrical (OE)conversion apparatus for data recovery. The bit error rate (BER) at theoptical receiver for a particular optical channel will depend upon thereceived optical power, the optical signal-to-noise ratio (OSNR),non-linear fiber effects of each fiber link, such as chromaticdispersion (CD) and polarization mode dispersion (PMD), and whether aforward error correction (FEC) code technique was employed in thetransmission of the data.

The optical power in each channel is naturally attenuated by the opticalfiber link or spans over which the channel signals propagate. The signalattenuation, as measured in dB/km, of an optical fiber depends upon theparticular fiber, with the total loss increasing with the length ofoptical fiber span.

As indicated above, each optical fiber link typically introduces groupvelocity dispersion (GVD) comprising chromatic dispersion (CD) andpolarization mode dispersion (PMD). Chromatic dispersion of the signalis created by the different frequency components of the optical signaltravel at different velocities in the fiber. Polarization modedispersion (PMD) of the signal is created due to the delay-timedifference between the orthogonally polarized modes of the signal light.Thus, GVD can broaden the width of an optical pulse as it propagatesalong an optical fiber. Both attenuation and dispersion effects canlimit the distance that an optical signal can travel in an optical fiberand still provide detectable data at the optical receiver and bereceived at a desired BER. The dispersion limit will depend, in part, onthe data rate of the optical channel. Generally, the limiting dispersionlength, L, is modeled as decreasing inversely with B, where B is the bitrate.

The landscape of optical transport networks has change significantlyover the past ten years. Prior to this time, most long haultelecommunication networks were generally handled via electrical domaintransmission, such as provided through wire cables, which is bandwidthlimited. Telecommunication service providers have more recentlycommercially deployed optical transport networks having vastly higherinformation or data transmission capability compared to traditionalelectrical transport networks. Capacity demands have increasedsignificantly with the advent of the Internet. The demand forinformation signal capacity increases dramatically every year.

In a conventional long haul DWDM optical network, erbium doped fiberamplifiers (EDFAs) may be employed at intermediate nodes in the opticalspan to amplify attenuated optical channel signals. Dispersioncompensation devices may also be employed to compensate for the effectsof fiber pulse dispersion and reshape the optical pulses approximatelyto their original signal shape.

As previously indicated, a conventional DWDM optical network requires alarge number of discrete optical components in the optical transmitterand receiver as well as at intermediate nodes along the optical linkbetween the transmitter terminal and the receiver terminal. Moreparticularly, each optical transmitter typically includes asemiconductor laser source for each optical channel. Typically apackaged module may include a semiconductor laser and a monitoringphotodiode (MPD) to monitor the laser source wavelength and intensityand a heat sink or thermal electric cooler (TEC) to control thetemperature and, therefore, wavelength of the laser source. The lasersources as well as the optical coupling means for the output light ofthe laser source to fiber pigtail, usually involving an optical lenssystem, are all mounted on a substrate, such as a silicon microbench.The output of the laser pigtail is then coupled to an externalelectro-optical modulator, such as a Mach-Zehnder lithium niobatemodulator. Alternatively, the laser source itself may be directlymodulated. Moreover, different modulation approaches may be employed tomodulate the external modulator, such as dual tone frequency techniques.

The output of each modulator is coupled via an optical fiber to anoptical combiner, such as, an optical multiplexer, for example, asilica-based thin film filter, such as an array waveguide grating (AWG)fabricated employing a plurality of silicon dioxide waveguides formed ina silica substrate. The fibers attached to each device may be fusionspliced together or mechanically coupled. Each of these device/fiberconnections introduces a deleterious, backward reflection into thetransmitter, which can degrade the channel signals. Each opticalcomponent and fiber coupling also typically introduces an opticalinsertion loss.

Part of the cost of the optical transmitter is associated with therequirement that the optical components also be optically compatible.For example, semiconductor lasers typically produce light output thathas a TE optical mode. Conventional optical fibers typically do notpreserve optical polarization. Thus, optical fiber pigtails andmodulators will transmit and receive both transverse electric (TE) andtransverse magnetic (TM) polarization modes. Similarly, the opticalcombiner is polarization sensitive to both the TE and TM modes. In orderto attenuate the effects of polarization dispersion, the modulator andthe optical combiner are, therefore, designed to be polarizationinsensitive, increasing their cost. Alternatively, polarizationpreserving fibers may be employed for optically coupling each lasersource to its corresponding modulator and for coupling each modulator tothe optical combiner. Polarization preserving fibers comprise fiberswith a transverse refractive index profile designed to preserve thepolarization of an optical mode as originally launched into a fiber. Forexample, the fiber core may be provided with an oblong shape, or may bestressed by applying a force to the fiber to warp the refractive indexof the waveguide core along a radial or cross-sectional lateraldirection of the fiber, such as a PANDA™ fiber. However, polarizationpreserving fibers are expensive and increase packaging costs since theyrequire highly accurate angular alignment of the fiber at each couplingpoint to an optical component in order to preserve the initialpolarization of the channel signal.

A conventional optical receiver also requires a plurality of discreteoptical components, such as an optical demultiplexer or combiner, suchas an arrayed waveguide grating (AWG), optical fibers, opticalamplifiers, and discrete optical detectors as well as electronic circuitcomponents for handling the channel signals in the electrical domain. Aconventional optical amplifier, such as an EDFA, has limited spectralwidth over which sufficient gain can be provided to a plurality ofoptical signal channels. Consequently, intermediate OEO nodes will berequired comprising a demultiplexer to separate the optical channelsignals, photodetector array to provide OE conversion of the opticalsignals into the electrical domain, 3R processing of the electricalchannel signals, EO conversion or regeneration of the processedelectrical signals, via an electro-optic modulator, into opticalsignals, optical amplifiers to amplify the channel signals, dispersioncompensators to correct for signal distortion and dispersion, and anoptical multiplexer to recombine the channel signals for propagationover the next optical link.

There is considerable interest in DWDM systems to increase both the datarate of each signal channel as well as the number of channels,particularly within the gain bandwidth of the EDFA. However, increasingthe channel data rate necessitates increasing the number of intermediatenodes along the optical path to provide the required signal dispersioncompensation and amplification. Increasing the number of channelsrequires precise control of channel assignment and more precise controlover signal dispersion, which dramatically increases the complexity andcost of the fiber-optic components of the system. A further complicationis that many pre-existing optical networks use different types ofoptical fibers in the different optical links of the optical networkhaving, therefore, different dispersion effects over different fiberlengths. In some cases, the wavelengths of the optical channelsgenerated at the optical transmitter may not be optimal for one or moreoptical links of the optical span.

What is desired are improved techniques to provide DWDM optical networkservices through improved, integrated optical network components andsystems.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an optical transmitter ortransceiver that comprises a PIC with integrated active and passivecomponents adapted to generate and/or receive optical channel signalsapproximately conforming to a standardized wavelength grid, such as theITU wavelength grid.

It is another object of the present invention to provide a method forin-wafer testing of monolithic integrated circuits (PICs) such asoptical transmitter monolithic integrated circuits (TxPICs) while in asemiconductor wafer.

SUMMARY OF THE INVENTION

According to this invention, a method of in-wafer testing is providedfor a monolithic photonic integrated circuit (PIC) formed in asemiconductor wafer where each such in-wafer circuit comprises two ormore integrated electro-optic components, one of each in tandem forminga signal channel in the circuit. The method includes the provision of afirst integrated photodetector at a rear end of each signal channel anda second integrated photodetector at forward end of each signal channel.Then, the testing is accomplished, first, by sequentially operating afirst of a selected channel electro-optic component in a selectedcircuit to monitor light output from a channel via its firstcorresponding channel photodetector and adjusting its operatingcharacteristics by detecting that channel electro-optic component outputvia its second corresponding channel photodetector to provide firstcalibration data. Second, by sequentially operating a second of aselected channel electro-optic component in the selected circuit tomonitor signal output from the second selected channel electro-opticcomponent via its second corresponding channel photodetector andadjusting its operating characteristics by detecting that channelelectro-optic component output via its second corresponding channelphotodetector to provide second calibration data. The first and secondcalibration data for each circuit channel for the selected circuit arethen stored for future reference.

More particularly, according to this invention, a method of in-wafertesting of monolithic transmitter photonic integrated circuits (TxPICs)formed in a semiconductor wafer is provided where each such circuitcomprises an array of integrated laser sources and an array ofintegrated electro-optic modulators, one each for each of the lasersources, and an optical combiner to receive the modulated signal outputsfrom each of the electro-optic modulators. The method includes the stepsof providing a first integrated photodetector at a rear facet of each ofsaid laser sources in each circuit, providing a second integratedphotodetector between each of said modulators and said optical combinerin each circuit, sequentially operating each of the laser sources in aselected circuit to monitor light output from a laser source via itsfirst corresponding photodetector and calibrate its operatingcharacteristics by detecting the laser source output via its secondcorresponding photodetector thereby providing first calibration data,sequentially operating each of the modulators in the selected circuit tomonitor signal output from a modulator via its second correspondingphotodetector and adjusting its bias point of operation to achieveoptimum extinsion ratio and chirp thereby providing second calibrationdata, and thereafter storing the first and second calibration data foreach circuit for future reference.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference symbols refer to like parts.

FIG. 1 is a schematic block diagram of an example of a single channel ina TxPIC chip.

FIG. 2 is another schematic block diagram of another example of a singlechannel in a TxPIC chip.

FIG. 3 is another schematic block diagram of a further example of asingle channel in a TxPIC chip.

FIG. 4 is a cross-sectional view of a first embodiment of a monolithicTxPIC chip illustrating a signal channel waveguide through an integratedDFB laser, EAM modulator and an optical combiner.

FIG. 5 is a cross-sectional view of a second embodiment of a monolithicTxPIC chip illustrating a signal channel waveguide through an integratedDFB laser, EAM modulator and an optical combiner.

FIG. 6 is a cross-sectional view of a third embodiment of a monolithicTxPIC chip illustrating a signal channel waveguide through an integratedDFB laser, EAM modulator, semiconductor optical amplifier (SOA) and anoptical combiner.

FIG. 7A is a schematic diagram of the plan view of a monolithic TxPICadapted also to receive data from an optical link.

FIG. 7B is a schematic diagram of a modified version of the monolithicTxPIC of FIG. 7A.

FIG. 7C is a schematic diagram of a further modified version of themonolithic TxPIC of FIG. 7A.

FIG. 8 is a schematic diagram of a plan view of a monolithic TxPIC forutilizing an on-chip photodetector to monitor facet reflectivity duringthe antireflection (AR) coating process.

FIG. 9 is a schematic diagram of a plan view of a first type ofmonolithic transceiver (TRxPIC) with interleaved optical transmitter andreceiver components.

FIG. 10 is a schematic diagram of a side view of a second type ofmonolithic transceiver (TRxPIC) useful for 3R regeneration and flip chipcoupled to a submount with control electronic semiconductor chipcomponents for operating the TRxPIC.

FIG. 11 is a schematic diagram of a plan view of a monolithic TxPIC withexternal monitoring photodiodes (MPDs) for monitoring the wavelengthand/or intensity of the laser sources.

FIG. 12 is a schematic diagram of a plan view of a monolithic TxPIC withdetachable integrated MPDs and heater sources provided for each lasersource and the optional SOAs, and for the optical combiner.

FIG. 13 is a schematic diagram of a plan view of a monolithic TxPIC withMPD coupled between each laser source and electro-optic modulator tomonitor the output intensity and/or wavelength of each laser source.

FIG. 14 is a schematic diagram of a plan view of a monolithic TxPIC withMPD coupled between each electro-optic modulator and the opticalcombiner to monitor the output intensity and/or chirp parameter of eachmodulator.

FIG. 15 is a schematic diagram of a plan view of a monolithic TxPIC withMPD coupled to a tapped portion of the multiplexed signal output of theTxPIC to monitor the signal channel intensity and wavelength.

FIG. 16 is a schematic diagram of a plan view of a monolithic TxPICsas-grown in an InP wafer.

FIG. 17 is a flowchart of a method for generating calibration dataduring manufacture to store calibrated data in adjusting the bias of thelaser sources, modulators and SOAs, if present, in the TxPIC andthereafter adjust the wavelength of the channels to be set at thepredetermined wavelengths after which the SOAs, if present, may befurther adjusted to provide the appropriate output power.

FIG. 18 is a schematic diagram of a plan view of another embodiment of aTxPIC chip where additional SMLs are formed at the edges of the InPwafer or, more particularly, to the edges of the TxPIC chip or die inorder to maximize chip yield per wafer.

FIG. 19A is a schematic diagram of a plan view of another embodiment ofa TxPIC chip where additional redundant SML sets are formed between SMLsets that are to be deployed for signal channel generation on the chipand used to replace inoperative SMLs, either at the time of manufactureor later in the field, thereby maximizing chip yield per wafer.

FIG. 19B is a schematic diagram of a plan view of another embodiment ofa TxPIC chip where additional redundant laser sources are provided foreach signal channel on the chip so that if one of the pair of lasersources is inoperative, either at the time of manufacture or later inthe field, the other source can be placed in operation, therebymaximizing chip yield per wafer.

FIG. 20 is a schematic diagram of a plan view of another embodiment of aTxPIC chip illustrating one embodiment of the provision of RF conductivelines employed for modulating the electro-optic modulators on the chip.

FIG. 20A is a graphic illustration of how the modulators of FIG. 20, orany other modulator in other embodiments, are operated via negative biasand peak-to-peak swing.

FIG. 21 is a perspective view of a schematic diagram of the biascontacts and bonding wire or tape for electro-optic components and theRF lines and contacts for the electro-optic modulators.

FIG. 22 is a schematic side view of a probe card with multiple probesinline with contact pad on a TxPIC chip to provide PIC chip testing atthe wafer level or after bum-in for reliability screening prior to finalchip fabrication.

FIG. 23 is flowchart of a method for wafer level testing of laser sourceoutput power using integrated PDs which may later be rendered opticallytransparent.

FIG. 24 is a schematic diagram of a plan view of another embodiment of aTxPIC chip illustrating the geometric arrangement of optical componentsto insure that stray light from the SML components do not interfere withthe output waveguides of the optical combiner.

FIG. 25 is a schematic diagram of a plan view of another embodiment of aTxPIC chip deploying Mach-Zehnder Modulators (MZMs) in the TxPIC chip.

FIG. 26 is a cross-sectional view of an embodiment of a DFB laser sourcethat may be deployed in FIG. 25.

FIG. 27 is a cross-sectional view of an embodiment of a Mach-ZehnderModulator (MZM) that may be deployed in FIG. 25.

FIG. 28 is a schematic block diagram of another embodiment of a singlechannel in the TxPIC chip of FIG. 25.

FIG. 29 is a schematic block diagram of a further embodiment of a singlechannel in the TxPIC chip of FIG. 25.

FIG. 30 is a graphic illustration of an example of the absorption of amodulator verses wavelength.

FIG. 31 is a cross-sectional view of an example of a band-edgeelectro-absorption modulator (BE-EAM).

FIG. 32 is a diagrammatic side view of multiple TxPICs with the samewavelength grid output but having separate TEC control to achieve awavelength band shift of one PIC relative to the other to achieve aseparate set of signal signals within the wavelength grid of the opticalcombiner.

FIG. 33 is a representative example of the multiple wavelength outputsof the pair of TxPIC chips of FIG. 32.

FIG. 34 is a schematic diagram of a plan view of an embodiment of anoptical transmitter portion of an optical transport system employing aplurality TxPIC chips with interleaved signal channel outputs.

FIG. 34A is a graph illustration of the first and second TxPICs of theoptical transmitter of FIG. 34 showing their wavelength outputs versepower before interleaving with a wavelength grid at a larger spatialseparation or pitch.

FIG. 34B is a graph illustration of the first and second TxPICs of theoptical transmitter of FIG. 34 showing their interleaved wavelengthoutputs verse power after interleaving with a wavelength grid at asmaller spatial separation or pitch.

FIG. 35A is an illustration of one kind of interleaving where the TxPICssuch as shown in FIG. 34 have on-chip channel spacing of 100 GHz or 200GHz.

FIG. 35B is an illustration of another kind of interleaving where theTxPICs such as shown in FIG. 34 have on-chip channel spacing of 50 GHz.

FIG. 36 is a schematic diagram of a plan view of an embodiment ofoptical transport system employing a plurality TxPIC chips withmultiplexed signal channels at the optical transmitter launched on afiber link and received at an optical receiver where the signal channelsare de-interleaved and demultiplexed to a plurality of RxPIC chips.

FIG. 37 is a schematic diagram of a plan view of a TxPIC chip with awavelength locker system utilizing frequency tone identifying tags foreach laser source in the TxPIC.

FIG. 38 is a graphic illustration of a frequency tone for a laser sourcein the TxPIC shown in FIG. 35.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIGS. 1A and 1B which illustrate, in blockform, an optical path on a monolithic TxPIC chip 10 showing pluralactive and passive optically coupled and integrated components. What isshown in diagrammatic form is one channel of such a chip. Both FIGS. 1Aand 1B show modulated sources coupled to an optical combiner. Shown inFIG. 1A is one of an array of sources comprising a directly modulatedsemiconductor laser 12 integrated with an optical combiner 16 having anoptical output waveguide 18 to take a combined channel signal off-chip.Shown in FIG. 1B is one of an array of sources comprising asemiconductor laser 12 optically coupled to one of an array ofmodulators comprising an electro-optic modulator 14 optically coupled toan input of an optical combiner 16 with the output of combiner 16coupled to an optical output waveguide 18. There are plural opticalpaths on chip 10 of semiconductor laser 12 and electro-optic modulator14, also in combination referred to as an SML, these SMLs respectivelycoupled to inputs of optical combiner 16. This is the basic monolithic,generic structure of a TxPIC chip 10 for use in an optical transmittermodule, also referred to by the applicants herein as a DLM (digital linemodule).

The semiconductor laser 12 may be a DFB laser or a DBR laser. While thelater has a broader tuning range, the former is more desirable from thestandpoint of forming an array of DFB lasers 12 that have peakwavelengths, which are created in MOCVD employing SAG (selective areagrowth) techniques to approximate a standardized wavelength grid, suchas the ITU grid. There has been difficulty in the integration of DFBlasers with an optical combiner but the careful deployment of SAG willprovide a TxPIC 10 that has the required wavelength grid. Thus, theoptical SML paths, mentioned in the previous paragraph, are modulateddata signal channels where the modulated channel signals arerespectively on the standardized grid. Electro-optic modulators 14 maybe EAMs (electro-absorption modulators) or MZMs (Mach-Zehndermodulators). Optical combiner 18 may be comprised of a star coupler, aMMI coupler, an Echelle grating or an arrayed waveguide grating (AWG).one of an array of sources. To be noted is that there is an absence inthe art, at least to the present knowledge of the inventors herein, ofthe teaching and disclosure of an array of modulated sources andwavelength selective optical multiplexer, e.g., such as an arrayedwaveguide grating (AWG) or Echelle grating. In this disclosure, awavelength selective multiplexer or combiner is defined as one that hasless than 1/N insertion loss wherein N is the number of modulatedsources being multiplexed. One principal reason is that it is difficultto fabricate, on a repeated basis, an array of lasers with a wavelengthgrid that simultaneously matches the wavelength grid of the wavelengthselective combiner (e.g., an AWG). The AWG is preferred because it canprovide a lower loss multiplexing structure. Additionally, an AWG mayprovide a narrow passband for grid wavelengths of lasers such as DFBlasers.

In FIG. 2, there is shown a further embodiment of a monolithic TxPIC 10chip. The TxPIC chip here is the same as that shown in FIG. 1B exceptthere is an additional active component in the form of semiconductoroptical amplifier (SOA) 20. Due to insertion losses in the opticalcomponents on the chip 10, particularly at points of their coupling, anon-chip amplifier 20 may be included in each EML optical path to boostthe output channel signals from modulators 14. An advantage of SOAs onTxPIC chips 10 compared to their deployment on RxPIC chips is therelaxation of the optical signal to noise ratio (OSNR) on the TxPIC SOAscompared to their employment in RxPIC SOAs, SOAs deployed on RxPIC chipsare positioned at the input of the chip to enhance the gain of theincoming multiplexed channel signal and is dominated by ASE generatedfrom the SOA which can effect the proper detection of channel signaloutputs. This is not as significant a problem in TxPIC chips whichrenders their usage in TxPIC chips as more acceptable in design freedom.As a result, the noise figure design criteria are relaxed in thetransmitter side, compared to the receiver side and being sufficient for100 km optical fiber link. Thus, OSNR limited optical devices can drivethe architecture and this has not been recognized by those skilled inthe art. More details of RxPIC chips can be found in U.S. patentapplication, Ser. No. 10/267,304, filed Oct. 8, 2002 and published onFeb. 19, 2004 as Pub. No. US2004/0033004A1, incorporated herein by itsreference.

It should be noted that the peak wavelengths of the SOAs 20 on a TxPICchip 10, such as, for example, SOAs 20 following each modulator 14 ofeach channel on a N channel TxPIC chip 10, should preferably have a peakwavelength slightly longer, such as, for example, in the range of 10 nmto 80 nm or preferably in the range of 30 nm to 60 nm, than itscorresponding semiconductor laser, such as a DFB laser, in order tocompensate for band-filling effects in SOAs 20, which effectively shiftsthe gain peak of an SOA 14 to shorter wavelengths when the SOA is placedinto operation. The amount of wavelength shift depends upon the designedbias point of the SOA. A preferred way to accomplish a different peakwavelength in SOAs 20, compared to its corresponding semiconductor DFBlaser, is to change the size or thickness of the active region of SOA 20to change its built-in peak wavelength through the use of SAG or,alternatively, through multiple layer regrowths. The use of SAG infabrication of chip 10 is discussed in more detail in U.S. patentapplication, Ser. No. 10/267,346, filed Oct. 8, 2002 and published onMay 1, 2003 as Pub. No. US2003/0081878A1, which application isincorporated herein by its reference.

Also, attention should be drawn to the optimization of active and activeoptical component spacing relative to substrate thickness to minimizethermal cross-talk between active optical components on TxPIC chip 10.Inter-component spacing of active optical components, such as DFB lasers12, modulators 14 and SOAs 20, is, in part, driven by thermal crosstalk,e.g., changes in temperature operation of these components that affectthe optical characteristics of neighboring active optical components,such as their wavelength or their bias point. Therefore, these activeoptical components should be sufficiently spaced in order to minimizethermal crosstalk affecting neighboring component operation. Componentseparation also important with respect to with substrate thickness.Ideally, the thickness of the substrate should be kept to a maximum inorder to minimize wafer breakage, particularly in the case of highlybrittle InP wafers, as well as breakage at the chip level duringhandling or processing. On the other hand, the substrate should not betoo thick rendering cleaving yields lower or resulting in excess heatingand thermal crosstalk due to thicker substrates. As an example, for a500 μm thick InP substrate, a preferred inter-component separation is inthe range of about 200 μm to about 600 μm.

Reference is now made to FIG. 3 which shows, in block form, a TXPIC chip10 similar to the chip shown in FIG. 1 except the output waveguide 18Afrom the optical combiner includes in its path an SOA. Thus, themultiplexed channel signals may be on-chip amplified prior to theirlaunching on an optical transport medium such as an optical fiber link.This chip output amplifier may be preferred as a gain-clamped SOA whichis discussed in more detail in connection with FIG. 9.

Reference is now made to cross section views of various representativeembodiments of a TxPIC chip 10. These cross-sectional views are not toscale, particularly in reference to the active waveguide core 42 of thedisclosed semiconductor chips. Chips 10 are made from InP wafers and thelayers are epitaxially deposited using an MOCVD reactor and specificallycomprise DFB lasers 12, EAMs. As seen in the cross-sectional view ofFIG. 4, there is shown an optical EML path and optical combiner of TxPICchip 10, comprising an InP substrate 32, such as n-InP or InP:Fe,followed by a cladding layer 34, a waveguide layer 36, a spacer layer 38of n-InP, followed by grating layer 40. Grating layer 40 includes agrating (not shown) in the section comprising DFB laser 12, as is wellknown in the art, having a periodicity that provides a peak wavelengthon a standardized wavelength grid. Grating layer 40 is followed by layer41 of n-InP, multiple quantum well region of wells and barriersemploying a quaternary (Q) such as InGaAsP or AlInGaAs. Thesequaternaries are hereinafter collectively referred to as “Q”. Theselayers are deposited deploying SAG using a mask to form the individualDFB bandgaps of their active regions as well as the bandgaps for theindividual EAMs 14 so that wavelengths generated by the DFB laser 12will be transparent to the individual EAMs 14. Also, the wavelength ofthe field of combiner 18 will be shorter than that of the EAMs 14. As anexample, the longest wavelength for a DFB array may be 1590 nm, its EAMwill have a wavelength of 1520 nm and the field of optical combiner 18will have a wavelength of 1360 nm.

The Q active region 42 and the waveguide core 36 layer extend throughall of the integrated optical components. If desired, the laser, and theSOA 20, if present, can be composed of a different active layerstructure than the region of the EAM 14. In this embodiment, the Qwaveguiding layer 36 provides most of the optical confinement andguiding through each section of the chip 10.

The chip 10 is completed with the growth of NID-InP layer 44, claddinglayer 46, which is either n-InP or NID-InP, and contact layer 48comprising p⁺⁺-InGaAs. Cladding layer 46 as well as its overlyingcontact layer portion is selectively etch away either over the EMLs orover the field of optical combiner 18 and regrown so that the partitionresults in p-InP layer 46A and p++-InGaAs layer 48A in regions of DFBlasers 12 and EAMs 14 and a NID-InP layer 46B and a passivation layer48B in region of the field of optical combiner 18. The reason for thisetch and regrowth is to render the optical combiner field 18non-absorbing to the optical channel signals propagating thought thisoptical passive device. More is said and disclosed relative to thismatter in U.S. application, Ser. No. 10/267,346, incorporated herein byits reference.

Chip 10 is completed with appropriate contact pads or electrodes, thep-side electrodes 44 and 46 shown respectively for DFB laser 12 and EAM14. If substrate 32 is semiconductive, i.e., n-InP, then an n-sideelectrode is provided on the bottom substrate 32. If substrate 32 isinsulating, i.e., InP:Fe, the electrical contact to the n-side isprovided through a via (not shown) from the top of the chip down ton-InP layer 34. The use of a semi-insulating substrate 32 provides theadvantage of minimizing electrical cross-talk between opticalcomponents, particularly active electrical components in aligned arrays,such as DFB lasers 12 and EAMs 14. The inter-component spacing betweenadjacent DFB laser 12 and EAMs 14 be about 250 μm or more to minimizecross-talk at data rates of 10 Gbit per sec.

Reference is now made to FIG. 5 which is the same as FIG. 4 except thatQ waveguide layer 36 is epitaxially positioned above active region 42rather than below this region as shown in FIG. 4.

Reference is now made to FIG. 6 which is similar to FIG. 4 except that,in addition, discloses an integrated optical amplifier comprising SOA 20with its p-side contact pad 49 and a spot size converter 22 formed inthe waveguide 18 from the optical combiner 18. To be noted is that theselective area growth (SAG) techniques may be employed to vary theepitaxial growth rate along the regions of the PIC to vary the thicknessof quantum well active layers longitudinally along the optical EML pathsof these optical active components. For example, in the case here,layers 42A in the active region 41 of EAM 14 are made thinner comparedto the DFB and optical combiner regions so that the optical modeexperiences tighter confinement during modulation with no probablecreation of multi-modes. Thus on either side of EAM 14, there are modeadaptors 14X and 14Y formed through SAG that respectively slightlytighten the confinement of the optical mode and permit slight expansionof the optical mode in the optical combiner where the propagation doesbecome multi-modal.

In SSC 22 of TxPIC chip 10 of FIG. 6, in region 42B of the active region42, the layers become increasingly narrower so that the optical mode inthe case here can expand more into NID-InP layer 46B permitting the modeexpansion to more approximate the numerical aperture of a couplingoptical fiber. In this connection, other layers of the structure may beshortened, such as in a step-pad manner as is known in the art, to forman aperture in the waveguide 18 from the PIC that provides a beam fromchip 10 to approximate the numerical aperture of a coupling opticalfiber.

TxPIC chip 10 is fabricated through employment of MOCVD where, informing active region 42 across all of the chips in an InP wafer, apatterned SiO₂ mask is positioned over the growth plane of the as-grownInP substrate. The patterned SiO₂ mask has a plurality of openings ofdifferent widths and masking spaces of different widths so that thegrowth rates in the mask openings will depend upon the area (width) ofthe opening as well the width of masks on the sides of the openings. Thereason that the mask widths play a role in what is deposited in theopenings is that the reactants, such as molecules of Ga and In, inparticular In, breakup or crack from their carrier gas quickly atregions of the SiO₂ mask and will migrate off the mask into the maskopenings. For example, quantum well layers grown in wider open areastend to grow slower and have a different composition than quantum wellsgrown on narrower open areas. This effect may be employed to varyquantum well bandgap across the plane of the substrate for each of theDFB lasers 12, EAMs 14 and the field of the combiner 18. Thecorresponding differences in quantum well energy can exceed 60 meV,which is sufficient to create regions having a low absorption loss atthe lasing wavelength. The SiO₂ masks are removed after the growth ofactive region 42. Additional growth and a subsequent etchback andregrowth are then performed, as previously discussed, to form acontinuous buried waveguide integrated transmitter chip.

An optical transport module may be fabricated employing a separate RxPICchip and a TxPIC chip. However, a TRxPIC chip is employed that includesboth transmitter and receiver components. The transmitter and receivercomponents share a common AWG or may be two AWGs, a first AWG for thetransmitter portion of the TRxPIC and a second AWG for the receiverportion of the TRxPIC. In this case, the AWGs may be mirrored imagedAWGs as known in the art. Embodiments of TRxPICs 10 are disclosed inFIGS. 7A through 8.

Reference is first made to FIG. 7A illustrating an embodiment of TRxPICchip 10. Chip 10 comprises an array of DFB lasers 12 and array of EAMs14 optically coupled via waveguides 24 to an optical combiner 18comprising an arrayed waveguide grating (AWG) 50. As, an example, TRxPICmay have ten signal channels with wavelengths of λ₁ to λ₁₀ forming afirst wavelength grid matching that of a standardized wavelength grid.However, as indicated before, the number of channel signal EMLs may beless than or greater than ten channels, the latter depending upon theability to spatially integrate an array of EMLs with minimal cross-talklevels. AWG 50 is an optical combiner of choice because of itscapability of providing narrow passbands for the respective channelsignals thereby providing the least amount of noise through itsfiltering function. Also, AWG 50 provides for comparative low insertionloss. AWG 50, as known in the art, comprises an input slab or free spaceregion 52, a plurality of grating arms 56 of predetermined increasinglength, and an output slab or free space region 54. AWG 50 is capable ofproviding for transmission of multiplexed channel signals as well as toreceive multiplexed channel signals. In this case, there are waveguides26A and 26B coupled between the output slab 54 of AWG 50 and the outputof chip 10. Output waveguide 26A is the output for multiplexed channelsignals 27 generated on-chip by the EMLs and launched onto the opticallink, and input waveguide 26B is the input for multiplexed channelsignals 29 received from the optical link. To be noted is that TRxPICchip 10 includes an array of integrated photodiodes (PDs) 15, two ofwhich are shown at 15A and 15B, for receiving incoming demultiplexedchannel signals on optically coupled waveguides 24 from AWG 50. Thus,AWG 50 is optically bidirectional and may be deployed simultaneously tomultiplex outgoing optical channel signals to output waveguide 26A andto demultiplex (route) a multiplexed input optical signal, preferablycomprising channel signals of different wavelengths from the outgoingchannel signals, which are coupled from the optical link fordistribution and detection to PDs 15A, 15B, etc. Thus, AWG 50 canfunction in one direction as a multiplexer and in the opposite directionas a demultiplexer as is known in the art. PDs 15 may be integrated PINphotodiodes or avalanche photodiodes (APDs). There may be, for example,an array of ten such PDs 15 integrated on TRxPIC 10. The electricalchannel signals generated by PDs 15 are taken off-chip for furtherprocessing as known in the art. It is preferred that the EML inputs fromwaveguide 24 to slab 52 of AWG 50 as well as the outputs from slab 52 toPDs 15 are formed in the first order Brillouin zone output of slab 52.

Alternatively, it should be noted that the input signal to TRxPIC 10 maybe one or more service channel signals, for example, from anotheroptical receiver or TRxPIC transmitter. AWG 50 would route these signalsto appropriate in-chip photodetectors 15 and taken off-chip aselectrical service signals for further processing.

In the embodiments herein deploying an AWG as an optical combiner, theAWG may be designed to be polarization insensitive, although this is notcritical to the design of the TxPIC 10. In general, an AWG does not needto be polarization insensitive because the propagating polarizationmodes from the respective DFB laser sources to the AWG are principallyin the TE mode. However, due to multimode propagation in the AWG, the TMmode may develop in one or more arms of the AWG in a worst casesituation. There are ways to combat this issue which are to (1) employpolarization selective elements, (2) place a TM mode filter at theoutput of the AWG and/or (3) make the SOAs 20, such as in the case ofthe embodiment of FIG. 6, have the same polarization bias as the DFBlasers 12 so that the amplification provided by the SOAs, followingmodulation, will amplify the TE mode rather than the TM mode so that anyamount of presence of the TM mode will be substantially suppressedbefore the TE mode encounters the AWG 50.

The design of the passive output waveguide 26A of AWG 50 of TRxPIC chip10, or any chip 10 embodiment output waveguide disclosed herein,involves several additional considerations. The total power coupled bythe AWG output waveguide 26 into optical fiber link should be sufficientto allow low error rate transmission. It is, thus, desirable that theoutput waveguide have a low insertion loss to increase the coupledpower. However, it is also desirable that the power density in the AWGoutput waveguide 26 be below the threshold limit for two photonabsorption. For an AWG output waveguide, such as waveguide 26, thiscorresponds to approximately 20 mW total average power for all channelsfor a waveguide width in the range of approximately 1 μm to 3 μm.Additionally, it is also desirable that output waveguide 26 be orientedat an angle relative to an axis perpendicular to the plane of the outputface or facet of chip 10, such as at an angle of about 7°, to reduce thecapture of stray light emanating from the on-chip EMLs in order tomaintain a high extinction ratio for signal channels. More will be saidabout this issue in connection with the embodiments of FIGS. 24A and24B.

Reference is now made to FIG. 7B which discloses the same TRxPIC 10 ofFIG. 7A except that the TRxPIC 10 of FIG. 7B includes, in addition, thearray of SOAs 58A, 58B, etc. formed in the on-chip optical waveguides 24to PDs 15A, 15B, etc. SOAs 58 respectively provide gain to demultiplexedchannel signals that have experienced on-chip insertion loss through AWG50 so that a stronger channel signal is detected by PDs 15. SOAs 58 areoptional and can be eliminated depending upon the design of AWG 50 whereit provides a low insertion loss, such as below 3 dB. TRxPIC 10 in bothFIGS. 7A and 7B include, as an example, ten signal channels withwavelengths of λ₁ to λ₁₀ forming a first wavelength grid matching thatof a standardized wavelength grid. The wavelength grid for receivedchannel signals may be, for example, λ₁₁ to λ₂₀ forming a secondwavelength grid matching that of a standardized wavelength grid. It ispreferred that the incoming channel signals be of different gridwavelengths so as not to provide any interference, particularly in AWG50. Compare this embodiment of FIG. 7B with the embodiment shown in FIG.8 to be later discussed. In the case here of FIG. 7B, the wavelengths ofthe incoming signals are different from the outgoing signal, whereas inFIG. 8 the wavelengths of the incoming and outgoing channels areinterleaved. In either case, the received channels, λ₁₁-λ₂₀, that areprovided as an output from the AWG may be coupled into SOAs 58.Furthermore, an optional SOA 59 may be integrated in the input waveguide26B before the input of AWG 50, a shown in FIG. 7B, to enhance theincoming multiplexed signal strength prior to demultiplexing at AWG 50.

Reference is now made to FIG. 7C which discloses a TRxPIC 10 that isidentical to that shown in FIG. 7A except that chip includes integratedmode adaptors or spot size converters (SSCs) 62 and 64 respectively inwaveguides 26A and 26B at the output of the chip for conforming theoptical mode of the multiplexed signals from AWG 50 to better fit thenumerical aperture of optical coupling fiber 60 and for conforming theoptical mode of the multiplexed signals from fiber 60 to better fit thenumerical aperture of chip 10 as well as waveguide 26B.

Another alternative approach for a TRxPIC 10 is illustrated in FIG. 8,which is basically the same as TRxPIC 10 of FIG. 7B except there areless transmitter and receiver channels, for example, only sixtransmitter channels and six receiver channels are disclosed, and theintegrated receiver channels are interleaved with the integratedtransmitter channels. Also, a single output waveguide 26 is for bothreceived and transmitted channel signals for chip 10. Chip 10 also has again-clamped semiconductor optical amplifier (GC-SOA) 70 instead of aSOA. GC-SOA 70 is preferred, particularly for received channel signal29, not only for providing on-chip gain to these signals but also thegain clamped signal or laser signal eliminates the loss of gain tohigher wavelength channels. Further, the TE/TM gain ratio of themultiplexed signal traversing the GC-SOA 70 is fixed due to the presenceof the gain clamped signal. Also, GC-SOA 70 provides gain to theoutgoing multiplexed channel signals, λ₁-λ₁₀. More about the utility ofGC-SOAs is found in U.S. patent application, Ser. No. 10/267,304,incorporated herein by its reference. A single AWG 50 is employed forboth the transmitter and receiver channels, which signal channels haveinterleaved wavelength bands. The channel wavelength band for thetransmitter channels are λ₁-λ₆, whereas the channel wavelength band forthe receiver bands are λ₁+Δ-λ₆+Δ where Δ is a value sufficient to notcause significant cross-talk with the transmitter channels. A GC-SOA isrequired in this embodiment as a non-clamped SOA will result insignificant cross-talk and pattern dependent effects. Furthermore, it islikely that the power levels of the incoming 29 and outgoing 27 channelswill be significantly different resulting in gain compression of thehigher power signals. Thus, a GC-SOA is required for the practicalimplementation of an on-chip amplifier in the location shown in FIG. 8.

Manufacturing variances in waveguide layer thicknesses and gratingperiodicity can cause significant variance in emission wavelength of DFBlasers fabricated on the same wafer and substantial lot-to-lot variance.Depending upon the fabrication process employed, the absolute accuracyof the DFB/DBR wavelength may be greater than about 1 nm due to theempirical process variances. For a single discrete DFB laser, control ofheat-sink temperature permits tuning to within less than 0.1 nm.Consequently, it is desirable to monitor and lock the emissionwavelength of each DFB laser in the array of the TxPIC to its assignedchannel wavelength while also maintaining the desired output power ofeach channel. The light output of at least one laser may be provided asinput to a filter element having a wavelength-dependent response, suchas an optical transmission filter. The optical output of the filter isreceived by an optical detector. Changes in lasing wavelength willresult in a change in detected optical power. The lasers are thenadjusted (e.g., by changing the drive current and/or local temperature)to tune the wavelength. If there are SOAs or PIN photodiodes on TxPIC 10integrated between the DFB lasers and the AWG in each signal channel,the SOA or PIN photodiode for each signal channel may be adjusted toadjust the relative output power levels to desired levels across thechannels.

Reference is made to FIG. 9 illustrating another embodiment, this timeof a TxPIC 10 which comprises only the transmitter channels of EMLs.Each EML optical channel comprises a DFB laser 12 and modulator 14 andAWG 50 of FIG. 7A, but having a single output waveguide 26 and onesingle photodiode PD 15T optically coupled by a waveguide 24 to theinput slab 52 of AWG 50. PD 15T may be coupled at the second orderBrillouin zone of slab 52 rather than the first order Brillouin zonewhere all the signal channels are coupled into slab 52. The applicationhere of PD 15T is different from the previous embodiments in that it isdeployed to check parameters on the chip after manufacture such as theamount of reflected light occurring within chip 10. In fabricating aTxPIC chip, it is often necessary to AR coat one or more facets of thechip, such as facet 10F of chip 10 where an AR coating 51 is place onthis output facet to prevent facet reflections of light back into chip10 from interfering with the multiplexed output signal. When an AWG 50is involved, the second order Brillouin zone, PD 15T on the input sideof AWG 50 may be utilized to monitor this reflected light from facet10F. PD 15T is operated as facet 1OF is being AR coated, i.e., in situ,or employed as a check of facet coating reflectivity after the ARcoating has been completed. During in situ use, when a desired minimumreflection is detected by PD 15, the AR coating process is terminated,the desired thickness of the AR coating having been achieved. Also, PD15T may be deployed later in field use as a trouble shooting means todetermine if there are any later occurring internal reflections orundesired light entering the chip from the optical link interfering withits operation.

As shown in FIG. 10, a TxPIC and a RxPIC are fabricated on a singlesubstrate with each having their separate AWGs. In this embodiment, theintegrated PICs can be utilized in a digital OEO REGEN as also explainedand described in U.S. patent application, Ser. No. 10/267,212, filedOct. 8, 2002 and published on May 29, 2003 as Pub. No. US2003/0099018A1,incorporated herein by its reference. In FIG. 10 an OEO REGEN 79comprises RxPIC 80 and TxPIC 10 integrated as single chip. As in pastembodiments, TxPIC 10 comprises an array of DFB lasers 12 and EAmodulators 14, pairs of which are referred to as EMLs. The outputs ofthe EMLs are provided as input optical combiner 18, such as, for examplean AWG or power (star) coupler. Optical combiner 18 has an output at 27for optical coupling to fiber link. RxPIC 80 comprises an opticalwavelength-selective combiner 82, such as, for example an AWG or Echellegrating, which receives an optical multiplexed signal 29 fordemultiplexing into separate wavelength grid channel signals which, inturn, are respectively detected at an array of photodetectors 84, suchPIN photodiodes, providing an array of electrical channel signals.

As noted in FIG. 10, the OEO REGEN 79 is flip-chip solder bonded to asubmount, including solder bonding at 86 for connecting the convertedelectrical signals to IC control chip or chips 94, via electricalconductors and conductive vias in and on submount 83. IC control chip orchips 94 comprise a TIA circuit, an AGC circuit, as known in the art,and a 3R functioning circuit for re-amplifying, reshaping and retimingthe electrical channel signals. The rejuvenated electrical channelsignals are then passed through submount, via electrical conductors andconductive vias in and on submount, to IC modulator driver 98 where theyare provided to drive EA modulators 14 via solder bonding at 90 viatheir coupling through conductive leads in or on submount 83. Further,IC bias circuit chip 96 provides the bias points for each of therespective lasers 12 to maintain their desired peak wavelength as wellas proper bias point for EA modulators 14 midway or along the absorptionedge of the modulators at a point for proper application thepeak-to-peak voltage swing required for modulation. As can be seen, theembodiment of FIG. 10 provides for a low cost digital regenerator forregeneration of optical channel signals that is compact and residesalmost entirely in the exclusive form of circuit chips, some electronicand some photonic. Such an OEO REGEN 79 is therefore cost competitive asa replacement for inline optical fiber amplifiers, such as EDFAs.

To facilitate microwave packaging, the OEO REGEN 79 is preferablyflip-chip mounted to a submount to form electrical connections to theseveral IC control chips. Also, note that IC control chips can beflip-chip bonded to OEO REGEN 79. Also, further note that the OEO REGEN79 may comprise two chips, one the TxPIC chip 10 and the other the RxPICchip 80.

Referring now to FIG. 11, there is shown another embodiment of a TxPICchip 100A wherein an array of PDs 101(1) . . . 101(N) is provided,separate and outside of chip 100A, where each PD 101 is opticallycoupled to a rear facet of a respective DFB laser 102(1) . . . 102(N).It can be seen that there are an integral number of optical channels,λ₁, λ₂, . . . λ_(n), on chip 100A, each of which has a different centerwavelength conforming to a predetermined wavelength grid. PDs 101 areincluded to characterize or monitor the response of any or all ofrespective on-chip DFB lasers 102(1) . . . 102(N). DFB lasers 102(1) . .. 102(N) have corresponding optical outputs transmitted on correspondingpassive waveguides forming optical paths that eventually lead to acoupling input of optical combiner 110. For example shown here, theoptical waveguides couple the output of DFB lasers 102(1) . . . 102(N,respectively, to an SOA 104(1) . . . 104(N), which are optional on thechip, an EA modulator 106(1) . . . 106(N) with associate driver 106A, .. . 106AN, an optional SOA 108(1) . . . 108(N) and thence opticallycoupled to optical combiner 110, which may be, for example, an AWG 50.Each of these active components 102, 104, 106 and 108 has an appropriatebias circuit for their operation. The output waveguide 112 is coupled toan output of optical combiner 110.

Optical combiner 110 multiplexes the optically modulated signals ofdifferent wavelengths, and provides a combined output signal onwaveguide 112 to output facet 113 of TxPIC chip 100A for opticalcoupling to an optical fiber (not shown). SOAs 108(1) . . . 108(N) maybe positioned along optical path after the modulators 106(1) . . .106(N) in order to amplify the modulated signals prior to beingmultiplexed and transmitted over the fiber coupled to TxPIC chip 100A.The addition of off-chip PDs 101(1) . . . 101 (N) may absorb some of thepower emitted from the back facet of DFB lasers 102(1) . . . 102(N),but, of course does not directly contribute to insertion losses of lightcoupled from the front facet of DFB lasers 102(1) . . . 102(N) to otheractive on-chip components. The utility of off-chip PDs 101(1) . . .101(N) is also beneficial for measuring the power of DFB lasers 102(1) .. . 102(N) during a calibration run, and also during its operation, inaddition to being helpful with the initial testing of TxPIC 100A.

In FIG. 11, cleaved front facet 113 of chip 100A may be AR coated tosuppress deleterious internal reflections. Where the off-chip PDs 101(1). . . 101(N) are designed to be integral with chip 100A, the employmentof an AR coating on front facet 113 may be unnecessary because much ofthe interfering stray light internal of the chip comes from the rearfacet of the lasers reflecting internally to the front facet 113. Aswill be appreciated by those skilled in the art, each DFB laser 102 hasan optical cavity providing light in the forward and rearwarddirections.

Conventional semiconductor laser fabrication processes for DFB and DBRlasers permits substantial control over laser wavelength by selecting agrating periodicity. However, variations in the thickness ofsemiconductor layers or grating periodicity may cause some individuallasers to lase at a wavelength that is significantly off from theirtarget channel wavelength. In one approach, each laser and itscorresponding SOAs are selected to permit substantial control of lasingwavelength (e.g., several nanometers) while achieving a pre-selectedchannel power.

The DFB laser may be a single section laser. Additionally, the DFB lasermay be a multi-section DFB or DBR laser where some sections areoptimized for power and others to facilitate wavelength tuning.Multi-section DFB lasers with good tuning characteristics are known inthe art. For example, multi-section DFB lasers are described in thepaper by Thomas Koch et al., “Semiconductor Lasers For Coherent OpticalFiber Communications,” pp. 274-293, IEEE Journal of LightwaveTechnology, Vol. 8(3), March 1990, which is incorporated herein by itsreference. In a single or multi-section DFB laser, the lasing wavelengthof the DFB laser is tuned by varying the current or currents to the DFBlaser, among other techniques.

Alternatively, the DFB laser may have a microstrip heater or otherlocalized heater to selectively control the temperature of the laser. Inone approach, the entire TxPIC may be cooled with a single TEC thermallycoupled to the substrate of the TxPIC such as illustrated in FIG. 12.FIG. 12 illustrates TxPIC chip 100B which is substantially identical tothe embodiment of FIG. 11 except includes, in addition, integrated PDs107(1) . . . (N) between modulators 106(n) . . . (N) and SOAs 108(1) . .. (N), device heaters 102A, 108A and 112 as well as PDs 101(1) . . .101(N) which, in this case are integrated on chip 100B. PDs 101 may bedeployed for initial characterization of DFB lasers 102 and thensubsequently cleaved away as indicated by cleave line 116. PDs 107 aredeployed to monitor the output intensity and modulator parameters suchas chirp and extinction ratio (ER).

The array of DFB lasers 102 may have an array bias temperature, T₀, andeach laser can have an individual bias temperature, T₀+T_(i) through theemployment of individual laser heaters 102A₁ . . . 102A_(N). In FIG. 12,there is shown a heater 102A₁ . . . 102A_(N) for each DFB 102 on TxPICchip 100B, and also a separate heater 111 for optical combiner 110 and aTEC heater/cooler 114 for the entire the chip. The best combination maybe a heater 102A for each respective DFB laser 102 and a chip TECheater/cooler 114, with no heater 111 provided for combiner 110. In thisjust mentioned approach, the TEC 114 may be employed to spectrallyadjust the combiner wavelength grid or envelope, and individual heaters102A of DFB lasers 102 are then each spectrally adjusted to line theirrespective wavelengths to the proper wavelength channels as well as tomatch the combiner wavelength grid. Heaters 102A for respective DFBlasers 102 may be comprised of a buried heater layer in proximity to theperiodic grating of each DFB laser, embodiments of which are disclosedand described in U.S. application, Ser. No. 10/267,330, filed Oct. 8,2002 and published on May 22, 2003 as Pub. No. US2003/0095736A1 which isand incorporated herein by reference. It should be noted that inemploying a chip TEC 114 in combination with individual heaters 102A forDFB laser 102, it is preferred that TEC 114 function as a primary coolerfor chip 100B be a cooler, rather than heater, so that the overall heatdissipation from chip 1003B may be ultimately lower than compared to thecase where TEC 114 is utilized as a heater to functionally tune thecombiner wavelength grid. Where TEC 114 functions primarily as a cooler,a spatial heater 11 may be suitable for tuning the wavelength grid ofcombiner while TEC 114 function as a primary cooler for chip 100B tomaintain a high level of heat dissipation. Then, individual DFB lasers102 may be tuned to their peak operating wavelengths and tuned to thecombiner grid.

Reference is now made to the embodiment of FIG. 13 illustrating TxPICchip 100C that is identical to chip 1000A in FIG. 11 except for heaters102, the addition of integrated PDs 105(1) . . . 105(N) positioned inEML optical paths between SOAs 104(1) . . . 104(N) and modulators 106(1). . . 106(N). SOAs 104 are disposed between DFB lasers 102 andmodulators 106 and PDs 105 are disposed between SOAs 104 and modulators106. In order to obtain the desired total output power from DFB lasers102, two alternatives are now described. First, initialization of lasers102, a bias voltage is applied to PDs 105 for purposes of monitoring theoutput of the DFB lasers 102, attenuation, α_(bias), of the photodiodesmay, themselves, result in an insertion loss. However, by adjusting thebias of SOAs 104, the total desired output power for a given EML stageof TxPIC chip 100C may be maintained. One benefit of PDs 105 is theprovision of dynamic on-chip feedback without necessarily requiringpre-existing calibration data. Another benefit of PDs 105 is theenablement of the gain characteristics of SOAs 104 to be discerned.Second, during normal operation of TxPIC chip 100C, PDs 105 can functionas passive components through the lack of any biasing, which, if biasexisted, would provide some attenuation, α_(bias). When PDs 105 functionmore like a passive device, e.g., with no applied reverse bias,insertion losses associated with such in-line PDs 105 may besubstantially eliminated. For many power monitoring application, PDs 105need not to be operated as a reverse biased device and can even beslightly or partially positive bias to minimize any residual insertionloss and render them more transparent to the light from DFB lasers 102.Alternatively, a small portion, such as 1% or 2%, of the light in theEML optical path may be tapped off by deploying PDs 105 that include ablazed grating in the active/waveguide core, where the light is takenoff-chip for other functions such as wavelength locking of lasers 102 oradjustment of the laser intensity. As in the previous embodiment ofFIGS. 11 and 12, PDs 105 may be a PIN photodiode or an avalanchephotodiode, where the former is preferred.

Thus, from the foregoing, it can be seen that during a test mode, priorto cleaving chip 100C from its wafer, PDs in FIG. 13 may operate as anin-line power taps of optical power from DFB lasers 102 to calibratetheir operating characteristics. As previously indicated, after TxPICchip 100C has been cleaved from its wafer, during its a normaloperational mode, PDs 105 may be operated to be optically transparent inorder to minimize their inline insertion losses, or may be slightlyforward biased to further minimize any residual insertion losses or maybe operated with selected reverse bias to adjust attenuation to adesired level.

Reference is now made to the embodiment of FIG. 14 illustrating TxPICchip 100D, which is identical to FIG. 12, except there are PDs 109following SOAs 108 in the optical paths, whereas in FIG. 12 PDs 107precede SOAs 108. PDs 109 are beneficial for characterizing the totalperformance of all optical components upstream of these PDs, and hence,can be deployed as monitors of the total channel power before combiner110. Furthermore, the insertion loss of optical combiner can becharacterized by utilizing PDs 105 in combination with an additionalphotodiode integrated on chip 100D in a higher order Brillouin zoneoutput of combiner 110 or positioned in the off-chip output 120 ofoptical combiner 120, as shown in FIG. 15.

Reference now is made to FIG. 15 illustrating TxPIC 100E, which isidentical to TxPIC 100B in FIG. 12 except that there is shown a fiberoutput 120 optically coupled to receive the multiplexed channel signalsfrom output waveguide 26 where a portion of the signals are tapped offfiber 120 via tap 122 and received by PD 124. PD 124 may be a PINphotodiode or an avalanche photodiode. As previously indicated, PD 124may be integrated in wafer. PD 124, as employed on-chip, may be employedfor testing the chip output prior to cleaving TxPIC chip 100E from itswafer, in which case the photodiode is relatively inexpensive tofabricate and would be non-operational or cleaved from the chip afteruse. PD 124 is coupled to receive a percentage, such as 1% or 2%, of theentire optical combiner output, permitting the optical powercharacteristics of TxPIC chip 100E to be determined during wafer leveltesting, such as for the purposes of stabilization of laser wavelengthsand/or tuning of the wavelength grid response of optical combiner 110 toreduce insertion losses.

It should be noted that both SOAs, such as SOAs 108, or photodetectors,such as photodiodes 109, can further serve as optical modulators or asvariable optical attenuators, in addition to their roles as monitors.Multiple of these functions can be performed simultaneously by a singlephotodetector, such as photodiode 124, or an integrated, on-chipphotodiode at a first or higher order output of the multiplexer, or thefunctions can be distributed among multiple photodetectors. On-chipphotodetectors can vary power by changing insertion loss and, therefore,act as in-line optical circuit attenuators. They also can be modulatedat frequencies substantially transparent to the signal channelwavelength grid with little effect to modulate data that is notnecessarily the customer's or service provider's data.

Additionally, optical combiner 110 may include integrated photodiodes atthe output of optical combiner 110 to facilitate in locking the laserwavelengths and/or tuning of the grid of optical combiner 110 to reduceinsertion losses. Additionally, PD 124 may be utilized to determine thehigh-frequency characteristics of modulators 106. In particular, PD 124and associated electronic circuitry may be employed to determine a biasvoltage and modulation voltage swing, i.e., the peak-to-peak voltage,required to achieve a desired modulator extinction ratio (ER) and chirpas well as to characterize the eye response of each modulator throughapplication of test signals to each of the EA modulators 106. The biasvoltage and voltage swing of the modulator may be varied. An advantageof having PD 124 integrated on chip 100E is that, after initial opticalcomponent characterization, the photodetector may be discarded by beingcleaved off TxPIC chip 100E. An arrangement where photodiodes are inintegrated at the output of combiner 110 on TxPIC chip is disclosed inFIG. 7 of U.S. patent application, Ser. No. 10/267,330, incorporatedherein by its reference. The ability to discard the photodetector hasthe benefit in that the final, packaged device does not include theinsertion loss of the photodetector formerly employed to characterizethe performance of the modulator during an initial characterizationstep.

Although particular configurations of SOAs and PDs are shown in FIGS.11-15, it will be understood by those skilled in the art that more thanone SOA may also be employed along any channel.

Referring now to FIG. 16, there is shown in-wafer, the chip die of TxPIC100B, although other embodiments of FIGS. 12 or 13-15 may be shown. Acombination of photodiodes, both those inline with EML channels, such asPDs 101 and 109, as well as those off-line, not shown, which may be usedto tap off optical power from an inline blazed grating PD or from tapoff from output 112. Photodiodes may be located in several locations inTxPICs 100E in order to perform either on-substrate testing or inlinetesting when TxPICs 100E is operating “on-the-fly”. Also, a probe testercan be utilized for testing the TxPICs. It should be noted that PDs 101at the rear facet of DFB lasers 102 may be left on the final cleavedT_(x)PIC chip and utilized during its operational phase to set, monitorand maintain the DFB and SOA bias requirements.

FIG. 17 discloses, in flowchart form, a procedure for adjustment of thewavelength of the channel lasers, set to a predetermined gridwavelength, after which the on-chip SOAs may be adjusted to providefinal appropriate output power. As seen in FIG. 17, first, a channel isselected at 130 in the TxPIC for testing. Next, the selected DFB laseris turned on and the output is checked via a photodiode, such as PDs 105in FIG. 13, to generate data and provide calibrated data (134) as towhether the laser wavelength is off from its desired grid wavelength andby how much. This calibrated data is used to adjust the laser wavelength(136) by current or heater tuning. If the desired wavelength is notachieved (138), the calibration process is repeated. The change inwavelength may also change the optical power available since the powervia applied current to the laser affects the amount of power. Ifoptimized wavelength and optical power adjustment is achieved (138),then SOA, such as SOAs 104, is adjusted (140) to provide to desiredoutput power for the laser. If all of the laser channels on the TxPICchip have not been tested (142), the next laser channel is selected(146) and the process is repeated at 132. When the laser channel hasbeen tested, the calibration data for all laser channels for the testTxPIC chip is stored at 144 for future use, such as for recalibrationwhen the transmitter module in which the TxPIC chip is deployed isinstalled in the field. The stored data functions as benchmark fromwhich further laser wavelength tuning and stabilization is achieved.

Reference is now made to FIG. 18 illustrating another configuration forTxPIC 10 deploying dummy optical components to the edges of a waferand/or edges of the PIC chips in order to maximize chip yield. Thesedummy components would be fabricated in the same way as the otheroptical components on the wafer using MOCVD. TxPIC 10 of FIG. 18comprises a plurality of DFB lasers 12 and EA modulators 14 formed asintegrated EML channels which are coupled to AWG 50 via integratedwaveguides 24. On adjacent sides of these optical components areadditional DFB lasers 12A and EA modulators 12A on one side andadditional DFB lasers 12B and EA modulators 12B on the other side. Theseadditional optical components are all shown as optically coupled to AWG50. However, they need not be so connected to AWG 50. Furthermore, it isnot necessary that bonding pads be connected to them. This will savechip space or chip real estate. The function of the dummy opticalcomponents is to take on the faulty attributes that occur to fabricateoptical components at edges of wafers or chips. It is problematic thatthe areas of component defects due to wafer fabrication, such as growthand regrowth steps, lithography, and other processing steps will likelybe at the edges of the wafer or boarder components on TxPIC chip edgeswhere these extra dummy optical components reside. By employing thesedummy components, the yield of useable wafers and good TxPIC chips willsignificantly increased.

Generally speaking from MOCVD fabrication experience as well as frombackend chip processing experience, the component yield on any PIC chipwith multiple optical components tends to decrease relative to eitheroptical PIC chips formed at the edges of the wafer or optical componentsformed along the edges of the PIC chip. There are several reasons forthis attribute. First, at the InP wafer level, an outer perimeter regionof the wafer tends to have the greatest material non-uniformity andfabrication variances. An edge region of a PIC may correspond to one ofthe perimeter regions of the wafer and, hence, also have suchsignificant variances. Second, the cleaving of the wafer produces thePIC dies. The cleaving process may adversely affect the edge opticalcomponents of the PIC die or these edge components may experience thegreatest amount of handling.

Statistical methods are employed to form a map of edge regions having areduced yield compared with a central region of a chip or die, or at thewafer level. The redundancy number of dummy optical components requiredin an edge region is selected to achieve a high yield of wafers where atleast one of the dummy optical components is operable for testing orreplacement of another failed component. As an illustrative example, ifthe yield in a central PIC region was 90% but dropped to 60% in an edgeregion, each dummy optical component in the edge region could includeone or more redundant optical components to increase the effective dummyoptical component yield to be at least comparable to the central region.It will also be understood that placing dummy optical components in edgeregions may be practiced in connection with previously describedembodiments.

To be noted is that the output waveguides 26 of AWG 50 in FIG. 18 is avernier output in the first order Brillouin zone output of AWG 50. Theoptimum waveguide among the several waveguides shown is chosen basedupon the waveguide exhibiting the best overall wavelength grid response.

It should be noted that with respect to the foregoing TxPIC chip andTRxPIC chip embodiments as well as, provision should be made forcircumvention of free carrier absorption due to two photon absorption inpassive waveguides 26 from AWG 50. The output waveguide length from theoptical combiner or AWG must allow sufficient output power to permit lowerror rate transmission but also must be below the limit for 2 photonabsorption. The 2 photon absorption limit is about 20 mW total averagepower for all signal channels for an approximately 1 μm to 3 μm wideoutput waveguide.

Two photon absorption can occur in passive waveguide structures,particularly if sufficiently long to induce photon absorption in theirwaveguide core. There are several ways to circumvent this problem.First, reduce the peak intensity in the waveguide, either transverselyor laterally or both. By rendering the mode to be less confined, i.e.,making the mode larger, the chance for the onset for two photonabsorption will be significantly reduced if not eliminated. Second, thepeak intensity of the optical mode may be shifted so as not to besymmetric within the center of the waveguide, i.e., the peak intensityof the mode is asymmetric with respect to the cladding or confininglayers of the guide as well as the center position of the waveguidecore. This asymmetry can be built into the chip during its growthprocess. Third, increase the Eg of core waveguides/cladding layers. Inall these cases, the point is to reduce the peak intensity in somemanner so that the threshold for two photon absorption is not readilyachieved.

Another approach to reduce or otherwise eliminate the free carrierabsorption due to two photon absorption is by hydrogenation of thewaveguides in situ in an MOCVD reactor or in a separate oven. Theprocess includes employing AsH₃, PH₃ and/or H₂ which creates H⁺ atomsites in the waveguide layer material during component fabrication whichdissipate or rid the waveguide of these absorption carriers.

Reference is now made to FIG. 19A illustrating another embodiment ofTxPIC, which in the case here includes an extra or dummy EML signalchannel beside each of the EML signal channels to be deployed foron-chip operation. As shown, extra DFB lasers 12EX and EA modulators14EX are formed on chip 10 adjacent to a corresponding laser 12 andmodulator 14 These sets of such lasers and modulators have the samebandgap wavelengths and lasing wavelengths. Thus, if a laser 12 ormodulator 14 in an operating set would fail, the adjacent laser 12EX andmodulator 14EX would be substituted in place of the failed EML channelset. Alternatively, it should be realized that, instead of functioningas replacement EML channel sets on chip 10, these extra EML channel setscan be deployed later, at an additional cost to the carrier provider, tofurther increase the signal channel capacity of the transmitter module.It should be realized that chip 10 can be made to include additionalcapacity not initially required by the service provider at a minimalcost of providing addition integrated EML channel sets on the chip whichcan be placed into operation at a later time. This is an importantfeature, i.e., the utilization of micro-PICs having multiple arrays ofEMLs fabricated on the same chip.

Reference is now made to FIG. 19B illustrating TxPIC chip 10 with pairsof DFB lasers 12A and 12B for each EML channel to provide redundancy onTxPIC chip 10. Each of the lasers 12A and 12B are coupled to anintegrated optical 2×1 combiner 13. Thus, the second DFB laser of eachpair 12A and 12B, can be placed into operation when the other DFB laserfails to meet required specifications or is inoperative. This redundancycan be applied to modulators 14 as well. This feature can be combinedwith the dummy optical component feature set forth in FIG. 19A.

Reference is now directed to the TxPIC chip 10 in FIG. 20 whichillustrates an embodiment of the contact layout strategy for EMLs on thechip. A multichannel TxPIC chip 10 has a substantial area compared to aconventional single semiconductor laser. Each optical signal source of aTxPIC requires driving at least one modulator section. Each modulatorsection requires a significant contact pad area for making contact to amicrowave feed. This creates potential fabrication and packagingproblems in routing microwave feeds across the substrate onto themodulator contact pads. As illustrated in the embodiment of TxPIC chip10 in FIG. 20, as an example, the location of contact pads 171 for themodulators may be staggered to facilitate microwave packaging. Microwavecontact pads 171 are coupled to modulators 14 for coupling RF signals tothe modulator electrodes. Chip 10 is shown with eight EML channelsoptically coupled to optical combiner 16 for multiplexing the channelsignals and placement on output waveguide 18 for coupling to an opticallink. The important feature is that the EA modulators 14 are staggeredrelative to one another along the optical path between respective DFBlasers 12 and optical combiner 16. The purpose for this arrangement isto provide for easier electrical contact directly to the modulators 14for signal modulation and bias. As shown in FIG. 20, co-planar microwavestriplines 170, 172 and 174 are fabricated on top of the chip to eachmodulator 14 from contacts 171, where lead 170 is connected to aprepared opening to p-contact 173 and coplanar leads 172 and 174 areconnected to a prepared opening to common n-contact 175. Contacts 175are connected to the n-side of the modulator through a contact viaprovided in the chip, such as down to n-InP layer 38 in the embodimentof FIG. 6. The p-contact pad is connected to the contact layer, such asto contact layer 48 in the embodiment of FIG. 6. The modulators 14 areelectrically separated from one another through etched channels preparedbetween the modulators which may extend down as far as the InP substrate32 as shown in the embodiment of FIG. 6. Also, a bias lead (not shown)is connected to the n and p contacts to provide a bias midpoint for thevoltage swing from peak-to-peak in modulation of the modulator. Also,bias leads 176 are also provided to each of DFB lasers 12 from edgecontact pads 170 provided along the rear edge of chip 10. Thus, contactpads 171 for modulators 14 are provided along two side edges of chip 10whereas contact pads 1070 are provided along one rear edge of chip 10for bias connection to DFB lasers 12 except that the centrally locatedmodulators 14 have their RF and bias contacts extend from the rear edgecontacts 170.

Pad staggering can also be accomplished in several different ways.First, additional passive waveguide sections are included to stagger thelocations of the optical modulators relative to a die or chip edge. Forexample, a curved passive waveguide section can be included in everyother DFB laser to offset the location of the optical modulator and itscontact pads. Second, the contact pads of modulator 14 are geometricallypositioned relative to the chip edges to be staggered so that straightleads can be easily designed to extend from edge contact pads to thestaggered modulator pads.

Reference is made to FIG. 20A which illustrates in graphic form thegeneral waveforms for modulation of modulators 14. In FIG. 20, there isline 180 which is zero bias. Modulators 14 are modulated with a negativebias to provide low chirp with high extinction ratio. Thus, the voltagebias, V_(B), is set at a negative bias at 182 and the voltage swing hasa peak-to-peak voltage, V_(PP), 184 within the negative bias range. Themodulation of modulator 14 according to a data signal illustrates thecorresponding modulator output at 186. One specific example of voltagesV_(B) and V_(PP) is a bias voltage of V_(B)=−2.5V and a swing voltage of2.5V or V_(PP)=−1.25V to −3.75V.

Reference is now made to the embodiment shown in FIG. 21 which is aperspective view of a TxPIC chip 10. The assembly in FIG. 21 comprises amulti-layer ceramic, or similar submount is utilized. As will be seen inthe description of this embodiment, a submount is mounted above TxPICchip 10 and in close proximity to the high-speed modulation pads onTxPIC chip 10. Transmission lines are formed on the submount. Microwaveshielding may be included above the submount. In order to ensure thatsufficient isolation is achieved between TxPIC 10 and the submount, anairgap, d, is formed between these two components, preferably where d isin a range of values around 5 mils or 127 μm.

Each of the optical modulators 14 of TxPIC chip 10 requires at least onemicrowave drive signal 200 and at least one common stripline 198.However, in the embodiment here, two common striplines 198 are utilizedto reduce crosstalk between the striplines of adjacent striplines to beconnected to adjacent modulators 14 on chip 10. RF striplines,comprising striplines 198 and 200, are formed on an array connectorsubstrate 195, which may be made of a ceramic material, which is spaced,such as by 50 μm, from TxPIC chip 10 as seen at 193. The forward ends ofstriplines 198 and 200 are respectively contacted to p-contact pads 173and common n-contact pads 175 by means of bonding wires 196B as shown inFIG. 21. Alternatively, these connections can be made by wire ribbonbonding or with a flexible circuit cable.

Chip 10 is supported on CoC submount 190 which includes patternedconductive leads 191 formed on a portion of the submount 190. Theseleads may, for example, be comprised of TiW/Au. Submount 190 may, forexample, be comprised of AlN. These patterned leads 191 end at contactpads 191A along the rear edge of chip 10. The bias signals provided onthese leads 191 are transferred to on-chip contact pads 12PD (which mayhave a 100 μm pitch on TxPIC 10) by means of a wire bonded ribbon 196A,or alternatively, a flexible circuit cable, where the respective ribbonleads are connected at one end to contact pads 191A and at the other endto contact pads 191B for DFB lasers 12. The additional patterned leadsare utilized for connecting to on-chip laser source heaters and on-chipmonitoring photodiodes.

An important feature of the embodiment of FIG. 21 is the deployment ofan L-shaped substrate 192 that has a thickness greater than that of chip10 so that the mounting of array connector substrate 195 on the top ofL-shaped substrate 192 will provide for the micro-spacing of around 5mils or 127 μm between chip 10 and substrate 195 so that no damage willoccur to chip 10, particularly during the backend processing ofconnecting conductor leads to chip 10. Thus, substrate 192 may becantilevered over chip 10 or a support post may be provided betweensubstrate 192 and substrate 195 at corner 199.

The assembly in the embodiment of FIG. 21 is concluded with top cover194 over substrate 195 which is micro-spaced from the top of substrate195 with spacer substrates 195A and 195B to provide spacing over RFstriplines 197. Cover 194 may be made of AlN or alumina and is providedfor a microwave protection shield for the micro-striplines 198 and 200as well as to provide structural support, particularly the suspendedportion of the assembly platform (comprising parts 195, 19X and 194)overhanging TxPIC chip 10 at 199. Cover 194 also includes cutout regions194A and 194B where cutout region 194B provides for tool access to makethe appropriate contacts 196B of the forward end striplines 198 and 200respectively to contact pads 175 and 173 of chip modulators 14. Therearward ends of striplines 198 and 200 are exposed by cutout region194A for off-chip assembly connection to a signal driver circuit asknown in the art.

A conventional alternative to the deployment microwave striplines 197 isto use wire bonding. However, it is not practical to use conventionalwirebonds to route a large number of microwave signals in a PIC. This isdue, in part, to the comparatively large area of the PIC that would berequired to accommodate all the wirebond pads and the wirebonds wouldhave to traverse a distance as long as several millimeters to reach allof the modulators. Also, the length of such wirebonds would create anexcessively large wire inductance and, therefore, would not be feasible.Additionally, the microwave cross-talk between the bonding wires wouldbe excessive. The high speed application required by TxPIC 10 for higherspeed data rates requires a transmission line with impedance matching tothe drive circuit which is difficult if not impossible to achieve withwire bonding. Thus, it is more suitable to deploy a flexible circuitmicrowave interconnect, such as at 196A, to couple RF or microwavestriplines 197 formed on substrate 195 to contact pads 173 and 175 ofeach modulator 14. A flexible microwave interconnect is an alternativeto wirebonds 196A for two reasons. First, they provide a reduction inassembly complexity. Second, they provide reduced inductance forwirebonds of equivalent length. A flexible circuit microwaveinterconnect is a microwave transmission line fabricated on a flexiblemembrane, e.g., two traces spaced apart to form a co-planar microwavewaveguide on a flexible membrane, that is at least one ground striplinefor each signal stripline. However, in the embodiment of FIG. 21, twoground striplines are shown which provides for less signal interferencedue to crosstalk with other tri-coplanar striplines. Each flexiblemicrowave interconnect at 196B would preferably have a contact portionat its end for bonding to contact pads 173 and 175 of a modulator 14using conventional bonding techniques, such as solder bonding,thermo-compression bonding, thermal-sonic bonding, ultra-sonic bondingor TAB consistent with wire ribbon bonding and/or flexible cableinterconnects.

It should be realized that TxPIC 10 may be flip chip mounted to asubmount, such as an alumina, aluminum nitride (AlN), or a berylliumoxide (BeO) submount. The submount is provided with patterned contactpads. In one approach, the submount includes vias and microwavewaveguides for providing the signals to the modulators. Conventionalflip chip soldering techniques are employed to mount the PIC electricalpads to the submount. The solder is preferably a solder commonly usedfor lasers, such as gold-tin, or lead-tin. A gold-goldthermo-compression bonding process may also be employed. Generalbackground information on flip-chip packaging technology is described inthe book by Lau, et al., Electronic Packaging: Design, Materials,Process, and Reliability, McGraw Hill, N.Y. (1998), which isincorporated herein by its reference. Some background information onmicrowave circuit interconnect technology is described in the book byPozar, Microwave Engineering, John Wiley & Sons, Inc. NY (1998).

There is a significant packaging cost associated with providing separateDC contact pads for driving each semiconductor laser, such as DFB lasersor DBR lasers. Driving the contact pads of groups of semiconductorlasers simultaneously reduces the number of DC pin outs and DCinterconnect paths required, which permits a substantial reduction inPIC area and packaging complexity, reducing PIC costs. As an example ofone approach, all of the DFB lasers 12 on a TxPIC 10 are driven inparallel. Alternatively, groups of lasers, e.g., three lasers, arecoupled in parallel. For multi-section lasers having a primary drivesection and a tuning section, the drive sections of groups of lasers maybe driven in parallel. Driving lasers in parallel reduces the packagingcost and the number of DC pin outs required. However, it also requiresthat the lasers have a low incidence of electrical short defects.Moreover, in embodiments in which groups of lasers are driven inparallel, it is desirable that the lasers have similar thresholdcurrents, quantum efficiencies, threshold voltages, and seriesresistances. Alternatively, the lasers may be driven in parallel, asdescribed above with the current to each laser being tuned by trimming aresistive element couple in the electrical drive line to the laser. Suchtrimming may be accomplished by laser ablation or standard waferfabrication technology etching. The former may occur in chip or waferform whereas the later is in wafer form. The trimming is done after theL-I characteristics are measured and determined for each laser.

Reference is now made to FIG. 22 which illustrates, in schematic form,the use of a probe card 200 containing a plurality of contact probes206A and 206B, such as, for example, one for each inline optical activecomponent, e.g., inline laser sources and their respective modulators,for each PIC chip to provide wafer level reliability screening before orafter wafer burn-in or die cleaving. The probe card 200 comprises a cardbody 202 which is supported for lateral movement over a PIC wafer bymeans of rod support 206. The top surface of probe card 200 includes aplurality of test IC circuits 204A and 204B which are connected, viaconnection lines 108A and 208B formed in the body of card 200, to aplurality of rows of corresponding contact probes 206A and 206B as shownin FIG. 22. Only six such contact probes 206A and 206B are seen in FIG.22 but the rows of these probes extend into the plane of the figure sothat there are many more contact probes than seen in this figure. Asufficient number of contact probes 206A and 206B are preferablyprovided that would simultaneously contact all contact pads on a singleTxPIC 10 if possible; otherwise, more than one probe card 200 may beutilized to check each chip 10. As seen in the example of FIG. 22, TxPICin wafer 11 includes rows of contacts 212 and 214, extending into theplane of the figure and formed along the edges of each TxPIC 10, therebysurrounding the centrally located, the formed active electro-optical andoptical passive components in region 210 internal of the chip 10. Probecard 200 can be laterally indexed in the x-y plane to test the PICs anddetermine their quality and their potential operability prior to beingcleaved from the chip. This testing saves processing time of latertesting of individual, cleaved chips only to find out that the chipsfrom a particular wafer were all bad.

With the foregoing processing in mind, reference is made to theflowchart of FIG. 23 illustrating a procedure for wafer level testingthe output power of the semiconductor lasers with inline, integrated PDswhich may later be rendered optically transparent when the PICs arecleaved from the wafer. As shown in FIG. 23, a probe card 200 iscentered over a PIC to be tested in wafer and brought into contact withits contact layers to first drive at least one of the semiconductorlasers 12 (220). Note, that a back or bottom ground contact may be alsomade for probe card testing. Next, a modulator 14 is driven with a testsignal (222). This is followed by setting the bias to the inline PD,such as PDs 105 and/or 109 in FIG. 16 (224). This is followed bymeasuring the power received by the PD (226) as well as measuring,off-chip, the operation of the laser, such as its output intensity andoperational wavelength (227). If required, the tested laser wavelengthis tuned (228). After all the lasers have been so tested, calibrationdata for each PIC on the wafer is generated (230) and stored (232) foruse in future testing before and after backend processing to determinedif there is any deterioration in the optical characteristics in any PIC.It should be noted that probe card 200 includes PIC identificationcircuitry and memory circuitry to identify each wafer level PIC as PICtesting is carried out so that the PICs tested can be easily lateridentified and correlated to the stored calibration data (232).

Reference is now made to FIGS. 24A and 24B which disclose TxPICarchitectures designed to minimize interference at the PIC outputwaveguide 26 of any unguided or stray light propagating within TxPICchip 10 and interfering with the multiplexed channel signals inwaveguide 26 thereby deteriorating their extinction ratio as well ascausing some signal interference. It should be noted that electro-opticintegrated components, particularly if SOAs are present, produce straylight that can propagate through the chip. It can be particularlydeleterious to the multiplexed output signals, deteriorating theirquality and causing an increase in their BER at the optical receiver. InFIG. 24A, TxPIC 10 is similar to previous embodiments comprising anarray of EMLs consisting of DFB laser 14 and EA modulators 14 coupled,via waveguides 24, to AWG 50. In the case here, however, it is to benoted that the arrays of EMLs are offset from AWG 50 and, furthermore,there is provided an isolation trench 234, shown in dotted line in FIG.24A, to block any stray, unguided light from the EML arrays frominterfering with output waveguides 26.

FIG. 24B is an alternate embodiment of FIG. 24A. In FIG. 24B, theorientation of the active components of TxPIC chip 10 are such that boththe laser and modulator arrays are at 90° C. relative to the outputwaveguides 26 of AWG 50. This PIC architecture optimally minimizes theamount of unguided stray light that becomes captured by the AWG outputwaveguides 26 and, therefore, does not appear as noise on themultiplexed channels signals thereby improving the extinction ratio ofthe outgoing multiplexed signals on one or more waveguides 26. Theextinction ratio loss from this stray light may be as much 1 dB.Wavelength selective combiner 50 may also be an Echelle grating.

FIG. 24C is an alternate embodiment of FIG. 24B. In the case here,rather than deploy a selective wavelength combiner, such as AWG in FIG.24B, a free space or power combiner 50C is instead utilized. Theadvantages of using power combiner 50C is that its insertion lossrelative to frequency is not dependent on temperature changes orvariations that occur due epitaxial growth as in the case of awavelength selective combiner. However, it has significantly higherinsertion loss for multiple signal channels, which insertion loss isdependent of critical dimension variation. Such a power combiner isdesirable in systems implementation wherein the link budget is notlimited by the launch power. That is, the reach of the system decreasessub-linearly with the decrease in launched power from the TxPIC. Also,such a TxPIC minimizes the amount of required temperature tuning asthere is no need to match the grid of the combiner to that of the gridof the transmission sources.

FIGS. 25-29 disclose the deployment of Mach-Zehnder modulators 240 inTxPIC chip 10 in lieu of EA modulators 14. As previously described, inthe case where the lasers themselves are not directly modulated, eachsemiconductor laser source is operated CW with its output opticallycoupled to an on-chip optical modulator. A high speed optical modulatoris used to transform digital data into optical signal pulses, such as ina return-to-zero (RZ) or non-return-to-zero (NRZ) format. Opticalmodulation may be performed by varying the optical absorptioncoefficient in an EAM, relative to the absorption edge illustrated inFIG. 30, or refractive index of a portion of the modulator, such as aMach-Zehnder modulator (MZM) illustrated in FIG. 28.

In FIG. 25, TxPIC chip 10 comprises an array of DFB lasers 12respectively coupled to an array of Mach-Zehnder modulators (MZMs) 240.The outputs of MZMs 240 are coupled to an AWG 50 via waveguides 24 as inthe case of previous embodiments. As is well known in the art, each MZM240, such as best shown in FIG. 28, comprises an input leg 240C, whichincludes DFB laser 270 and may also include an on-chip SOA, which legforms a Y coupling junction to separate phase legs or arms 240A and 240Band an output leg having a Y coupling junction connecting the arms 240Aand 240B to output leg 240C, which includes a waveguide 272 to amultiplexer and also may optionally include an on-chip SOA. As seen inFIGS. 26-28, MZM 240 includes phase altering contacts 264A and 264B. Theoperation of MZM 240 is well known in the art.

FIGS. 26-28 disclose one example of an InGaAsP/InP-based MZM 240. Thestructure shown is epitaxially grown using MOCVD and comprises asubstrate 242 upon which is epitaxially deposited cladding layer 244 ofn-InP, followed by waveguide Q layer 246 of InGaAsP or AlInGaAs,followed by layer 248 of n-InP, which is followed by buffer layer 252 ofn-InP. Next is active Q layer 254 of InGaAsP or AlInGaAs, followed byepitaxial growth of layer 256 of NID-InP followed by cladding layer 258of p-InP. Then, an etchback is performed which is followed by a secondselective growth comprising cladding layer 260 of p-InP and contactlayer 261 of p⁺-InGaAs. This is followed by the deposit of a passivationlayer 262 which, for example, may be comprised of SiO₂. Next, p-sidecontacts 264A and 264B are formed, after a top portion of passivationlayer 262 is selectively etched away, as well as the formation of then-side contact 266. A similar MZM is shown U.S. Pat. No. 6,278,170,which patent is incorporated herein by reference. The principaldifference between the MZM shown in this U.S. patent and the MZM inFIGS. 26 and 27 is the presence in the embodiment herein of waveguide Qlayer 246.

By applying a voltage in at least one arm of the MZM, the refractiveindex is changed, which alters the phase of the light passing throughthat arm. By appropriate selection of the voltage in one or both arms, aclose to 1800 relative phase shift between the two light paths may beachieved, resulting in a high extinction ratio at the modulator output.As described below in more detail, MZMs have the advantage that theyprovide superior control over chirp. However, MZM modulators requiremore PIC area than EAMs and may require a somewhat more complicateddesign as well for high-speed modulation, such as 40 Gb/s or more.

Reference is now made to FIG. 29 which illustrates a modified form ofthe MZM 240 illustrated in FIG. 28. It is desirable in deploying a MZMas the modulator of choice to also provide means to prevent the“extinguished” or stray light from the modulator from deleteriouslycoupling into other optical components of the TxPIC chip or any otherPIC chip for that matter. This is because the “extinguished” light,i.e., light not leaving the exit port of the MZM due to destructiveinterference at its exit port, may couple into other nearby opticalcomponents, resulting in deleterious optical crosstalk. A variety oftechniques may be employed to suppress deleterious cross-talk associatedwith the “extinguished” light. For example, an absorber region may bedisposed in the substrate or in an extra arm provided on the MZM outputas illustrated in FIG. 29. In FIG. 29, an absorber region 278 ispositioned at the end of the extra output arm 276 of MZM 240X coupled atoutput coupling crosspoint 274. This absorber region 278, for example,may be composed of a semiconductor or non-semiconductor material.Alternatively, a higher order grating or other deflector, such as anangled facet, may be formed at region 278 to direct the “extinguished”light out of the chip or into proximity of a buried absorbing layer orregion. Furthermore, the placement of a monitor photodiode (MPD) at 278may be utilized at the end of extra arm 276 to serve the function of anabsorber and which can further provide the additional function of anoptical monitor of the optical parameters of the signal output of MZM240Z.

An EAM or MZM may be characterized by its extinction ratio, which isgoverned by its on/off ratio. A high extinction ratio increases thesignal-to-noise ratio (SNR) at the optical receiver such that a highextinction ratio is generally desirable in order to achieve a low biterror rate (BER) at a downstream optical receiver. A modulator shouldalso possess low insertion loss, IL_(out) (dB)=10 log₁₀ P_(out)/P_(in),corresponding to the loss between its input and output ports. Amodulator typically also has a chirp parameter, which expresses theratio of phase-to-amplitude modulation. The chirp parameter isproportional to the ratio: Δn/Δα, where Δn is the differential change inrefractive index and Δα is the differential change in absorption.

The modulator chirp may be adjusted to compensate for chromaticdispersion in the fiber link. Typically, a modulator having a negativechirp parameter is desirable in order to achieve a maximum transmissiondistance on standard optical fibers having negative chromaticdispersion. In chirping, the laser wavelength may move to theshort-wavelength side (negative chirp) or to the long wavelength side(positive chirp) as the amplitude of the output light is modulated viathe modulator. A negative chirp is desirable to suppress dispersioninduced broadening of optical pulses that occurs in a conventionaloptical fiber at certain wavelengths.

An electro-absorption modulator (EAM) has an optical absorption lossthat typically increases with an applied voltage. In an EAM, a biasvoltage may be selected so that the electro-absorptive material isbiased to have a high differential change in absorption loss formicrowave voltage inputs.

A Mach Zehnder phase modulator utilizes changes in refractive index inthe modulator arms to modulate a light source. A Mach Zehnder modulator,such as MZM 240 in FIG. 25, receives CW light and splits the lightbetween two arms 240A and 240B. An applied electric field in one or botharms creates a change in refractive index due to the shift in absorptionedge to longer wavelengths. In general, a band-edge MZ modulatorachieves large phase changes due to large absorption changes at the bandedge via the Kramers-Kronig relation. However, a non-band edge MZmodulator achieves its phase change via the electro-optic effect orbased on the Franz-Keldysh effect. At the output of the modulator, thetwo split signals are joined back together at the Y-shaped couplingsection, shown in FIG. 28, or a directional coupler shown in FIG. 29.Destructive interference results if the relative phase shift between thetwo signals is 180 degrees. At very high data rates, traveling wavetechniques may be used to match the velocity of microwave pulses in theelectrodes of a modulator to optical signal pulses.

As illustrated in FIGS. 30 and 31, an EA modulator is designed to haveappropriate wavelength shifts from the band edge 283 of the absorptioncurve 281, shown in FIG. 30, where the absorption shift or loss is aprimary consideration change at a given wavelength in achieving thedesired modulation effect while any changes in index in the material isimportant for chirp. This is in comparison to band edge (BE)Mach-Zehnder modulators which have a larger wavelength shift to considerfrom the band edge and where the change in refractive index in an arm ofthe modulator can be significant index change because it is a functionof changes in absorption at all wavelengths from the band edge. So in anEAM, the range of operation is designed to be in the region of greaterabsorption loss changes relative to the band edge whereas the BE-MZM canoperate in regions of much less absorption loss changes.

As seen in FIG. 30, the typical absorption edge curve 281 is shown inFIG. 30. The Y axis parameter of FIG. 30 is the a or absorption of theEAM modulating medium and the X axis of FIG. 30 is the wavelength. Theabsorption band edge 283 is where the absorption strongly changes withwavelength, i.e., for example, a high increase in absorption over arelative short range of wavelength change, which may be about 20 nm. Inoperation, the DC bias of the EAM is chosen such that the wavelength ofthe band edge is close to the wavelength of the DFB laser light so thata small modulating electrical field across the modulator produces alarge change in absorption.

As shown in FIG. 31, an electro-absorption modulator may be comprise aPIN photodiode structure that is reverse biased to create an electricfield across an active region which may be low bandgap material, such asa high refractive index Group III-V compound or may be comprised one ormore quantum wells of such material. The applied electric field shiftsthe absorption edge to longer wavelengths (lower energy). As shown inFIG. 31, the EAM 280 comprises a substrate 282 upon which is epitaxiallydeposited a cladding layer of n-InP 284, followed by a Q waveguide layer286, thence a cladding layer 286 of n-InP, followed by Q etch stop layer288 of InGaAsP or AlInGaAs. This is followed by the epitaxial deposit ofa NID-InP layer 290 and thence a multiple quantum well active Q region292 where the electro-optic effect takes place, followed by a claddinglayer of p-InP and contact layer 296 of p+-InGaAs. An etchback isperformed to form loaded rib ridge waveguide for EAM 280. The etchbackis performed to etch stop layer 288 forming a ridge waveguide thatincludes layers 290, 292, 294 and 296.

In quantum wells, the shift in absorption edge can be more pronouncedthan that in bulk layers due to quantum size effects. By appropriatelyselecting the band edge in the modulator to be above the absorptionedge, a large shift in refractive index is possible for quantum wellstructures. Details of designing quantum well structures for modulatorsare described in the book by Vladimir V. Mitin, et al., QuantumHeterostructures: Microelectronics and Optoelectronics, CambridgeUniversity Press, NY (1999).

The transmission lines used to couple microwave signals to the opticalmodulators are preferably impedance matched. This is particularlyimportant for traveling wave modulator embodiments that may require moremicrowave power due to the increased interaction length. Also, resistorsmay be integrated into the PIC and are coupled to each microwavetransmission lines to achieve impedance matching.

By varying the quantum well structure, the absorption edge may beshifted relative to the lasing wavelength to increase the relativeeffective absorption and changes in refractive index. Anelectroabsorption modulator is commonly operated in a regime in whichincreasing reverse bias voltage increases the absorption. Typically,quantum well electroabsorption modulators must be operated in a highabsorption region to obtain a negative chirp, leading to high insertionlosses. See, e.g., the article, “Design of InGaAsP Multiple Quantum-WellFabry-Perot Modulators For Soliton Control,” Robert Killey et al., pp1408-1414, IEEE Journal of Lightwave Technology, Vol. 17(8), August1999, which is incorporated herein by its reference. Also, an importantadvantage of an EAM, particularly relative to use in a PIC, is that itoccupies less space on a PIC chip than a MZM.

In contrast, in a phase modulator, such as a Mach Zehnder modulator, thereverse bias voltage may be selected for any voltage range over whichthere is a substantial change in refractive index. This permits thevoltage bias and voltage swing of a quantum well Mach Zehnder modulatorto be selected to achieve a negative chirp with a low insertion losscompared with an electro-absorption modulator. It will be understoodthat any known Mach Zehnder modulation technique may be employed,including both single-arm and two-arm modulation. However, using two-armmodulation of MZMs is desirable to control chirp. MQW Mach Zehndermodulators have the benefit that a controllable negative chirp may beachieved at a bias voltage for which insertion losses are acceptable.The modulator, for example, may be a band-edge Mach Zehnder modulator.In a band-edge MZM, the bandgap wavelength of the MZM arm sections isslightly shorter in wavelength than the channel wavelength. Theabsorption edge of a band-edge MZM is thus near the channel wavelengthsuch that comparatively small voltage swings are required to achieve alarge shift in refractive index. An advantage of band-edge MZMs is thatcomparatively small voltages and/or arm lengths are required due to thelarge refractive index shifts possible. However, each band-edge MZMmodulator requires that its band edge be selected to be close to thechannel wavelength of its corresponding laser. Multiple regrowths orselective area growth techniques may be used to adjust the band edgeenergy of each MZM relative to its corresponding laser.

Also, any known velocity-matched traveling wave modulator configurationmay be beneficially employed to improve the efficiency of the modulatorfor achieving high data rates. In a traveling wave modulator theelectrode of the modulator is used as a transmission line. In atraveling wave modulator the velocity of microwave signals travelingalong the modulator electrodes is preferably matched to the velocity oflight traveling along the optical waveguide of the modulator. Atraveling wave modulator has a high 3-dB bandwidth. Additionally, atraveling wave modulator may have a substantial optical interactionlength. The long potential interaction length of a traveling wavemodulator permits greater freedom in selecting a bias voltage andvoltage swing to achieve a controlled chirp, a high extinction ratio,and a low insertion loss.

The bias voltage of the modulator may be selected to achieve a negativechirp appropriate for a particular fiber link relative to its fiberlength and fiber type. Also, a different DC bias may be selected foreach modulator in the TxPIC chip. For example, an EA modulatorpreferably has a bandgap that is between about 20 nm to about 80 nmshorter in wavelength than that of its laser for optimal chirp andextinction ratio characteristics. In principle, each modulator couldhave an active region that is grown (using regrowth or selective arearegrowth) to have a predetermined difference in bandgap with respect toits laser. However, in a TxPIC for providing a substantial number ofchannel wavelengths, this may require a comparatively complicated growthprocess. It is preferable, in terms of device fabrication, to have asmall number of different active layer bandgaps. Consequently, it iswithin the scope of this invention to independently DC bias each on-chipmodulator to adjust its desired chirp characteristics. As anillustrative example, a 1V change in DC bias (e.g., from −2V to −3V) inan EAM can accommodate a DFB laser wavelength variation of about 25 nm.

It should be noted that it may be difficult, in some cases, to achievethe desired chirp, extinction ratio and insertion loss using thisbiasing technique. Thus, as discussed earlier, it may be necessary tovary both the peak wavelength of the laser array as well as that of eachmodulator. A preferred technique to realize such a laser array is withselective area growth (SAG), which is disclosed and discussed in U.S.patent application, Ser. No. 10/267,346, which is incorporated herein byreference. In a preferred selective area growth approach, a pattern ofmask openings is fabricated in SiO_(N) layer or another suitabledielectric material. The size of the mask openings and or the width ofthe masks forming the openings for the different DFB lasers, which areto be fabricated, are varied so that there is a resulting wavelengthvariation across the DFB array. Similarly, the modulator wavelength isvaried by having a larger opening (to create a larger bandgap)multiquantum well region that varies across the array. Note, however,that the DFB wavelength is ultimately determined by the grating pitch.The necessity for selective area growth across the array arises from theneed to shift the gain peak across the array. In general, better lasercharacteristics are obtained if the gain peak is in close proximately,e.g., within about 10 nm, or somewhat longer wavelength than the lasingwavelength selected by the grating. The placement of this peak does notrequire high precision. Thus, a different SAG window may not need to beemployed for each laser. The alignment of the modulator bandgap to thatof the laser is the more precise parameter, especially where the chirp,low insertion loss, and high extinction ratio are required. Thus, inalmost all cases, the openings of the SAG mask as well as the maskwidths will need to be varied across the array of modulators.

The extinction ratio of the modulator may be characterized during aninitial testing, such as by employing a PIC optical detection element toform eye diagrams as a function of the bias voltage and voltage swing ofthe modulator for a simulated series of modulator “ones” and “zeros.”The chirp may be characterized at the TxPIC chip level during testingemploying known techniques, such as by measuring the linewidth of aparticular channel as it is modulated. Calibration data of bias voltagesand voltage swings required to achieve a desired extinction ratio forselected chirp levels may be stored on a computer readable medium.Additionally, calibration data of insertion loss as a function ofmodulator parameters may also be acquired to permit the SOA drivecurrent and/or PIN photodiode bias to be correspondingly adjusted tomaintain a desired channel power as the modulator parameters are varied.As previously indicated, the calibration data for controlling modulatorand SOA and/or PIN photodiode parameters can be stored in a programmablememory, such as an EPROM, and packaged with the PIC for use by the enduser or customer.

The modulator operating parameters of bias voltage and voltage swing maybe controlled through feedback data received from an optical receivervia the optical link. In a high data rate channel close to thedispersion limit, a positive chirp increases the BER while a negativechirp decreases the BER. Similarly, a high extinction ratio tends todecrease the BER while a low extinction ratio tends to increase the BER.A forward error correction (FEC) chip in the optical receiver may beemployed to determine the BER of each signal channel. This informationmay be forwarded to the TxPIC transmitter in a variety of ways, such asthrough an electrical control line or through an optical servicechannel. The operating parameters of bias voltage and voltage swing ofthe modulator of a channel are adjusted using data received backrelative to its channel BER. Chirp control of the modulators is derivedfrom information received relative to the BER data from the receivercommunicated to the TxPIC transmitter or transceiver via an opticalservice channel. An electronic controller in the TxPIC transmitteremploys this data to tune the bias voltage and/or voltage swing of themodulator to adjust its chirp to achieve the desired BER based uponcharacteristics of a particular fiber type comprising the optical spanor link.

The chirp parameter of a quantum well EA modulator is a function of thechange in absorption characteristics and refractive index of themodulator with bias voltage. Typically, a voltage bias may be selectedover a range within which the chirp parameter shifts from positive tonegative. However, as previously indicated, it is preferred to operatewith negative bias voltage and negative swing to produce the best chirpwith the highest extinction ratio (ER) as indicated in connection withrespect to FIG. 20A.

As previously indicated, it is desirable to have a controlled chirpselected to achieve a maximum fiber transmission length appropriate forthe channel wavelength and the fiber type. One way to adjust thecharacteristics of the optical modulator is to select one or more layersin the absorber section to have a controlled absorption edge withrespect to the lasing wavelength. Methods to control the absorptioncharacteristics of the modulator as a function of applied electric fieldinclude using regrowth techniques to grow materials with selectedcomposition and thickness in the modulator region and using MOCVDselective growth techniques to grow quantum wells in the modulatorhaving a pre-selected difference in absorption band edge compared withthe laser section. Alternatively, the modulator may comprise cascaded ortandem electro-absorption modulators, one of which is illustrated inFIG. 14 of U.S. patent application, Ser. No. 10/267,330, filed Oct. 8,2002 and incorporated herein by its reference. A firstelectro-absorption modulator may be used to generate periodic string ofpulses at a clock frequency (e.g., 10 GHz). The pulses may be amplifiedby an on-chip SOA. A second electro-absorption modulator may be used toprovide a gating function to put data on the generated pulses. Onebenefit of this embodiment is that it permits the use of a RZ signalformat. Additionally, by appropriately setting the electro-absorptionmodulator parameters, a controlled chirp may be achieved. The SOAprovides compensation for the insertion loss of the modulator.

In another embodiment, a saturable absorber may be coupled to the outputof the modulator. In this case, a first modulator stage, such as amulti-section EA modulator, may be used to generate optical data pulses.An integrated saturable absorber section (SAS) receives the output ofthe first modulator stage and has non-linear transmission properties. Ifthe output of first modulator stage is low, corresponding to anoff-state, the SAS is absorptive, further decreasing the amplitude ofthe signal in the off-state. However, if the output of the firstmodulator stage is high, the absorption of the SAS saturates, resultingin comparatively low losses for the on-state. A benefit of employing aSAS is that it increases the extinction ratio of a modulator.

The SAS can be placed along the optical signal source path anywhereafter the modulator. For example, the SAS is placed immediately afterthe modulator or after a following on-chip SOA. An important benefit ofplacing the SAS downstream from the SOA is that it suppresses SOA ASEnoise for “zero” signals, resulting in an improvement of the OSNR. TheSAS is preferably fabricated from a quantum well active region that hassaturable losses at the channel signal wavelength. The SAS may be areverse biased, partially unpumped, or completely unpumped region. Anyknown technique to reduce the recovery time of the SAS may be employed,such as ion implantation. An unpumped SAS has the benefit of simplefabrication. However, a reverse biased SAS may provide more stableoperating characteristics for higher data rates and modulation.

Generally speaking, the design of the modulator may include theoreticalor empirical studies to select a quantum well structure having anabsorption edge that varies with applied voltage relative to the channelwavelength such that a desired extinction ratio and negative chirp maybe achieved. The extinction ratio and chirp effects depend also upon thebias voltage of the modulator, which should be set to achieve thedesired chirp with an acceptable insertion loss.

It should be understood that in connection with all of the modulatorembodiments described herein, a SOA within the optical signal path maybe employed to compensate for insertion loss associated with adjustingthe bias voltage of the modulator to achieve a desired chirp. Thepresent invention permits simultaneous wavelength locking, selection ofoutput channel power, and tuning of modulator operating characteristicsto achieve a desired extinction ratio and chirp. Also, if desired, anelectronic controller for the PIC may include calibration data and/orfeedback algorithms for regulating these parameters. The chirp parametermay be set in the factory or in the field.

Another feature herein is the employment of SAG for fabrication ofband-edge (BE) MZ modulators so that their size can be monotonicallychanged across the modulators in the modulator array to have appropriateabsorption curves relative to its respective laser source. The use ofSAG provides an approach where the size of such a modulator is reducedcompared to other types of modulators, since they are shorter in length,thereby taking up less area or real estate on the PIC chip. Such BE MZmodulators may be deployed in less costly TxPICs with tunable chirp.

Reference is now made to FIG. 32 which illustrates the temperaturetuning of different TxPIC chips 300 and 302 which have the samewavelength grid at room temperature. For the purposes of simplicity, itis assumed that each TxPIC 300 and 302 has been designed, employing SAGgrowth techniques, to have four DFB lasers with a wavelength grid of λ₁,λ₂, λ₃, and λ₄. As seen in FIG. 32, the desired wavelength grids ofmultiple TxPICs 300 and 302 are achieved by providing each TxPIC chipwith its own TEC 304 and 306, respectively. The tuning range for DFBlasers on these TxPIC chips is in the range of about 0.1 nm/° C. Thetemperature tuning range is typically about 10° C. to about 40° C., withwavelength tuning range, therefore, of about up to 3 nm for each lasersource. The tuning rate of the DFB lasers can be compared to AWGs 50which is about 0.11 nm/° C.

TxPICs 300 and 302 are tuned via TEC at T₁ so that the first TxPIC 300has a wavelength grid of λ₁, λ₂, λ₃, and λ₄ and tuned via TEC at T₂ sothat the second TxPIC 302 has a wavelength grid of λ₅, λ₆, λ₇, and λ₈,forming a total wavelength grid 308 as illustrated in FIG. 33. Thus, thesecond TxPIC 302 is tuned to a higher temperature (T₂>T₁) so that it hasa wavelength grid of longer wavelength channels where the wavelengthspacing relative to both chips may be at 100 GHz or 200 GHz. Aninterleaver, such as interleaver 318 shown in FIG. 34, may also becoupled to receive the channel outputs from TxPICs 300 and 302 where theinterleaver may have a smaller predetermined grid spacing, such as 50GHz. Temperature adjustment of the wavelength peaks and correspondinggrid of the respective TxPICs may be achieved such that the desired gridspacing is obtained and maintained at the interleaver.

Reference is now made to FIG. 33 which illustrates an example ofmultiple TxPICs 310 with multiple wavelength outputs optically coupledvia waveguides 316 to an 8×1 interleaver 318. Each of the TxPIC chips310 have a SML array 312 providing plural signal channels to opticalcombiner 314. As shown in the example of FIG. 34, there are eight TxPICchips 310 with the respective generated wavelength grids shown in FIG.34. TxPICs may be heated at different levels to each achieve desiredtuned wavelength grid relative to a standardized wavelength grid. Eachone of the chips 310 may have a wavelength grid spacing of 200 GHzwhereas the spacing of the interleaver wavelength grid may be 50 GHz.The configuration of heaters on each TxPIC chip 310 comprises individuallaser heaters and a PIC TEC cooler (not shown). The individual TxPICwavelength grids, two of which are shown in FIG. 34, can then be tunedto have the proper grid spacing relative to the interleaver wavelengthgrid so there is an interleaved grid spacing for the multipleinterleaved grid wavelengths of TxPICs 310 at 50 GHz as illustrated inFIG. 34B. The output from interleaver 318 is provided to link 328 viabooster amplifier 326, which may be an EDFA.

The output from interleaver 318 also includes a 2% tap 320 for divertinga portion of the output via fiber 321 to wavelength locker 322.Wavelength locker includes conversion of the optical signal into one ormore electrical signals, amplification of the electrical signals andpartitioning of the signals into a plurality of separate signalscorresponding to individual elements of the modulated sources, such asthe source wavelength. The locker 322 deploys signal filters for each ofthe wavelengths relative to each respective SML array wavelength grid todetermine if the respective grid wavelengths are off the desiredwavelength grid. If a wavelength deviates from the desired wavelength, acorrection signal is generated and transmitted, via a digital to analogconverter (DAC), to the respective TxPIC 310 for wavelength change vialines 323. The correction signal is employed at PIC circuitry at theTxPIC chip to either change the current applied to the laser source tochange the laser wavelength to the correct operating wavelength orchange the current applied to laser source heater to change thewavelength to the correct operating wavelength. Of course, other tuningmethods as known in the art may be utilized.

The interleaving as shown in FIG. 34B is discussed in more detail withrespect to FIGS. 35A and 35B. Relative to the employment of a pluralityof TxPIC chips in a transport network, the output interleaved channelspacing in an interleaver is equal to the initial channel spacingdivided by a power of two, depending upon interleaver design. FIGS. 35Aand 35B disclose respective systems of interleaving and multiplexingchannel signals. In general, the interleaving of different TxPIC chipwavelengths, as shown in FIG. 35A, allows for ease in the DFB tolerancesthereby avoiding the close on-chip wavelength spacing across the arrayspacing resulting in relaxing requirements imposed upon the fabricationof a wavelength selectable combiner. The system illustrated in FIG. 35Apermits the fabrication of four channel wavelength-different, fourchannel TxPICs 10 with grid wavelengths design, as shown from λ₁ to λ₁₆.As seen, on-chip grid wavelength spacing is easily achieved using a gridwavelength spacing of 200 GHz. With the deployment of interleaver 318the wavelength spacing between interleaved optical channels is 50 GHz,as shown in FIG. 35. This is an important feature since it has not beenknown to utilize a plurality of InP chips with multiple channels perchip, such as the multiple TxPICs 310 in FIG. 34, having a given numberof signal channels provided with a larger wavelength spacing, easing therequirements in the manufacture of the integrated combiners. Thewavelength grid required for the AWG is now larger, e.g., 200 GHz,instead of 50 GHz, where the number of grating arms of the AWG isinversely proportional to wavelength spacing so that fewer arms on theAWG are required as the wavelength spacing is increased. Fewer arms inan AWG translates to easier fabrication and potentially reduced AWG andchip size. Also, the epitaxial requirements are less stringent such asuniformity relative to composition and layer thickness and targetingrequirements in MOCVD growth are reduced. This, in turn reduces the costof manufacture of TxPICs by virtue of having a higher yield with moreacceptable TxPIC chips per wafer. In summary, multiple TxPICs withplural channels can be fabricated with less stringent tolerances,providing for higher chip yields per wafer, by having larger on-chipwavelength spacing between signal channels, such as 200 GHz. The TxPICchip outputs can sequentially be interleaved at smaller wavelengthspacing, such as 50 GHz. This interleaving of FIG. 35A is preferred forlong haul networks, having the advantage of tuning individual PICs tothe proper wavelength grid while reducing their fabrication tolerances.

The channel multiplexing system in FIG. 35B is possibly preferred formetro networks. Channel multiplexing in FIG. 35B provides forsequentially combined TxPICs. In this application, the wavelengths aretypically spaced further apart (e.g., about 200 GHz). This largerspacing results in reduced requirements for the DFB wavelengthtolerances required across the array as well as for the AWG tolerances,significantly reducing the cost of the TxPIC. Metro networks typicallydeploy less channels, and hence utilize wider channel spacing. Ifinterleaved TxPICs were utilized in conjunction with such channelspacing, the ability to fabricate these TxPICs would push the limits ofthe fabrication processes. For example, an interleaved channel spacingof 200 GHz requires that the channels for each TxPIC be on an 800 GHzgrid. Such large channel spacings are difficult for TxPICs that utilizelarger channel counts, e.g., 10 channels or more. Furthermore, a simplemultiplexing element is considerably less costly than an interleavingelement. Thus, a low-cost, low channel count system preferably utilizesmultiplexed TxPICs rather than interleaved TxPICs. Note here thatthe-TxPIC costs are also reduced in addition to the cost of the passiveoptical components. In FIG. 35B the multiple four channel TxPICs areinitially fabricated with a 200 GHz wavelength spacing on each chip andare multiplexed to provide wavelengths with 200 GHz spacing.

Reference is now made to FIG. 36 which illustrates an optical transportnetwork deploying a de-interleaver and red/blue demultiplexers. Forsimplicity of description only a unidirectional network is shown,although the principle explained can also be applied to a bidirectionalnetwork. The network of FIG. 36 comprises, on the transmit side, aplurality of TxPICs 310 (eight in the example here) each with an SMLchannel array (four signal channels shown here), as shown and describedin connection with FIG. 34, including feedback wavelength locker 322.The description of FIG. 34 also applies here, except that themultiplexed signal outputs from TxPICs 310 are provided to an 8:1multiplexer 317, rather than an 8:1 interleaver 314, for combining as anoutput on optical link 328. For example, the wavelength spacing of thechannel signals at TxPIC chips 310 may be 200 GHz for each TxPIC or4×200 GHz, and on link 328 may be 50 GHz for thirty-two combined channelsignals or 32×50 GHz via multiplexer 317.

At the receiver side, there is a group (eight in the example here) ofRxPIC chips 342 each comprising, at its input, an optical decombiner344, such as an AWG, and a plurality (four in the example here) ofphotodiodes, which may be PIN photodiodes or avalanche photodiodes. Tobe noted is the lineup of the RxPIC chips 340 on the receiver side isnot the same as the lineup of TxPICs 310 on the transmitter side, i.e.,the RxPIC chip lineup is RxPIC 1, 5, 2, 6, 3, 7, 4 and 8. Also, at theinput from optical link 328, there is a 4×1 de-interleaver 330 thatde-interleaves groups of channel signals into pairs of red/blue signalgroups corresponding to respective groups of channel signals at TxPICson the transmitter side. Thus, for example, the output on waveguide332(1) would be eight channels, or two groups each of four channels,with channel spacing of 200 GHz or 8×200 GHz. By rediblue groups, it ismeant groups of shorter and longer signal channels. Thus, again, in thecase of waveguide 332(1), the red group (relative to channels from TxPIC1) is λ₁-λ₄ (4×200 GHz) and the blue group (relative to channels fromTxPIC 5) is λ₁₇-λ₂₀ (4×200 GHz).

The advantage of the network deployment of FIG. 36, particularly on thereceiver side is that the employment of de-interleaver 330 reduces thenumber of grating arms required in the AWG decombiners 344 because thewavelength channels are divided into rediblues groups with largewavelength separations at 200 GHz. This eases the fabricationspecifications for RxPIC chips 340 reducing the requirements of thefiltering function of the AWGs 340. By reducing the number of AWGgrating arms, there is less concern about epitaxial uniformity acrossthe AWG field during MOCVD growth. Also, there is chance of producingphase errors because of greater distribution of the channels signalsthrough a greater number of grating arms. Thus, de-interleaver 330 ofFIG. 36 provides a narrow band filter which are of a relatively widepassband AWG 344 on each RxPIC 340. RxPIC AWGs 344 require stringentcrosstalk specifications for low noise output channel signals foroptimum detection at PD arrays 342. This leads to the use of moregrating arms utilized in AWGs 344, usually several more such gratingarms. Also with a tightening of channel spacing such as 100 GHz or even50 GHz requires additional grating arms for optimum filtering of thechannel signals. Thus, these two requirements increase the need foradditional grating arms. However, the deployment of interleaver 330 inFIG. 36 reduces these requirements on the number of grating arms forAWGs 344 since the channel spacing of channels reaching the AWG ormultiplexer is wider. Therefore, the filter passband of the wavelengthgrid of the AWGs can be wider, easing the fabrication requirements inthe design and growth of AWGs 344.

Also, the de-interleaver/channel RxPIC combination significantly reducescosts through the reduction in the number of required demultiplexers 334(only four instead of eight in the example here) as well as the numberof optical fiber connections. Relative to the concept of providing lessfiber connections in an optical transmitter module, note that the numberof demultiplexers in the embodiment here are cut in half and,correspondingly, also a number of fiber connections are cut in half.Further, four channels being integrated on each RxPIC chip translates toa four to one reduction in necessary fiber connections compared to theconventional deployment of discrete signal channel components presentlydeployed throughout today's optical transport networks.

The interleaver 318 and de-interleaver 330 are currently available indifferent forms such from JDS Uniphase, e.g., their IBC interleaver,e.g., 50/100 GHz or 100/200 GHz passive interleavers.

It is within the scope of this invention that the optical transportnetwork of FIG. 36 service, for example, both the L band as well as theC band. In this case, a C/L band demultiplexer would precedede-interleaver 330 to direct, for example, the C band channels to thisde-interleaver, while the L band channels would be directed to acorresponding L band de-interleaver (not shown) and a correspondingarray of RxPICs 340. Also an optical amplifier, such as a EDFA, may bepositioned between the C/L demultiplexer and the respective C band and Lband de-interleavers to provide gain to the channel signals. Such anoptical amplifier may also be utilized in the network of FIG. 36, beingpositioned just before the input of de-interleaver 330.

Reference is now made to FIG. 37 which illustrates a TxPIC 10 coupled toa low-cost wavelength locking system 350. As shown in FIG. 37, each DFBlaser source 12 has a laser driver 364. The approach of FIG. 37 ischaracterized by employing AWG 50 to wavelength lock the laser sourcearray 12, i.e., matching the wavelength grid of passband of AWG 50 tothe operating wavelengths of DFB lasers 12. The embodiment heredisclosed illustrates a TxPIC chip 10 with ten signal channels.Wavelength locking will allow for tighter signal wavelength channelspacing and more efficient use of the available optical spectrum. Themethod here utilizes unique identifying tags, such as different ditheror tone frequencies, associated with each DFB laser source 12. Thesetags can also be deployed for other purposes, such as, very low costper-channel power monitoring.

While tones have been chosen to illustrate a particular form of opticalmodulation useful for channel identification and signal processing forwavelength locking, other modulation formats such as multitone, spreadspectrum, square wave, tone burst, etc. are envisioned, depending onspecific signal processing requirements. Similarly, while the variableoptical attenuator role of the photodetectors has been discussed inconnection with equalization of optical channel powers emerging from theTx PIC, more general relationships among individual optical channelpowers are envisioned. In particular, pre-emphasis, i.e., thedeliberately arranging unequal individual optical channel powers fromthe transmitter to compensate for channel-dependent unequal losses intransmission links, is envisioned and enabled by the variable opticalattenuator function on individual optical channels.

It should be further noted that on-chip photodiodes can be deployed toencode the signal channel with additional information useful for signalchannel identification, wavelength locking, or data transmissionadditional to that encoded by modulators 14. As an illustration, onesuch photodiode can have its bias voltage modulated by a sine wave orsquare waves, unique to the particular optical channel, to label theoptical channel for use in channel identification and wavelength lockingwithout demultiplexing the optical channels. Other modulations (toneburst, spread spectrum, multitone, etc.) can be used similarly for thesepurposes. On-chip photodiodes can also be used as voltage variableoptical attenuators, useful for controlling individual optical channelpowers.

The passband of an optical component, such as an AWG, a WDM filter orfiber grating(s), in a transmitter TxPIC 10 can be employed also as away of directly locking the laser source wavelength or multiple lasersource wavelengths in the TxPIC transmitter to the passband of such anoptical component.

An AWG, for example, has a Gaussian passband for each laser sourcewavelength, and can be employed as a frequency differentiator in orderto lock the laser source wavelength directly to the AWG passband. Thelocking can be achieved by dithering the drive currents of thecorresponding laser sources at a low frequency, such as 1 KHz, 2 KHz . .. 10 KHz, one of which is illustrated at 370 in FIG. 38. A differentdither or tone frequency is provided for each DFB laser source 12 viatone frequency driver or generator 366 in each drive current path to DFBlasers 12. In FIG. 38, the frequency of the dither is indicated at 372and its amplitude is indicated at 374. The modulation depth 376 iscontrolled such that the laser source frequency shift is appropriate forthe AWG passband and control loop electronics of system 350, i.e. theresulting amplitude variations are just sufficient for the loopelectronics at 350 to comfortably distinguish the laser source tags fromone another.

The amplitude variations 374 resulting from dithering are low frequency(low KHz range) which can be ignored or may be filtered out at thenetwork optical receiver end and will have negligible impact on BER orjitter specifications, beyond the impact of lowering the average opticalpower at the receiver. The slow wavelength variations will not impactthe system performance since the instantaneous linewidth appears fixedfor any given large bit pattern, e.g., approximately 10⁶ bits forOC-192.

It is possible to utilize the method of stabilization of FIG. 37 toassign a different dither frequency to each laser source 12 on TxPICchip 10 so that a single tap 320 and photodetector 351 can providesufficient feedback for all DFB laser sources 12. Here, 1% tap coupler320 is placed after the output of TxPIC chip 10 and a singlephotodetector 351 is employed to simultaneously detect all ten signalchannels. The detected electrical signal is amplified via electricalamplifier 352. The ten different signal channels are then separated byelectronic filters 358(1) . . . 358(10), comprising 1 KHz filter 358(1),2 KHz filter 358(2) . . . 10 KHz filter 358(10), centered around each ofthe laser source tone frequencies. Low speed feedback circuitry 360 thencompletes the loop via feedback lines 362 to the respective DFB lasersource current drivers 364. Circuit 360 determines if the peakwavelength of the respective laser sources is off peak, and by how much,from a predetermined peak or off-peak wavelength desired for therespective laser sources. The information relating to predetermined peakor off-peak wavelengths is stored in memory in circuit 360 and isobtained through initial factory testing of the wavelengths of theindividual laser sources 12 relative to the passband of the wavelengthgrid of AWG 50. The digital values obtained for differences between theoff-set from the desired wavelength values for each laser source areconverted from digital format to analog format, via a digital-to-analogconverter (DAC), within circuitry 360, and provided to laser sourcecurrent drivers 364 for changing the drive current levels to DFB lasers12 to correspondingly tune and optimize their operating wavelengths tosubstantially match the wavelength grid of AWG 50. As mentioned inseveral previous embodiments, a TEC unit may be utilized with chip 10and/or a local heater may be employed for AWG 50. Also, instead of, orin addition to, adjusting driver current to laser sources 12, each ofthe laser sources 12 may be provided with an adjacent heater strip (notshown) to be employed to tune the wavelengths of the individual lasersources 12. In general, any known conventional tuning elements or methodmay be employed instead of heating. Other wavelength tuning elementsinclude: adding multiple sections to the laser and varying the currentin each section (including, phase tuning, which is the provision of aphase section in a DFB or DBR laser), vernier tuning where the bestpassband response is chosen from multiple outputs of the opticalmultiplexer, the use of coolers to tune the wavelength grid orindividual elements of the PIC, including TECs which are also shown inconnection with the embodiments herein, and stress tuning such asthrough the use of bi-metals. Thus, any wavelength tuning contemplatedherein comprises wavelength tuning controlled by changes in temperature,voltage and current, or bandgap.

The use of unique dither frequency “identifying tags” for each lasersource 12 also allows a single photodetector 351 and circuitry 360 toperform diagnostics on the transmitter TxPIC chip 10, such as insuringuniformity in power per channel. In the embodiment shown in FIG. 37, thePIC output power for each signal channel can be determined employing asingle photodetector as is used for the wavelength locking. The averagepower seen on each dither frequency “identifying tag” can be calibratedto the associated channel output power and the overall DC photocurrentcan be calibrated to the total PIC output power. The PIC per channellaunch power is one of the most important optical link diagnostics.

The concept of employing unique identifying tags for each laser sourcemay also be extended to cover multiple TxPICs, such as, to the array ofTxPIC chips 310 shown in FIG. 36, by employing different sets of ditherfrequencies for different PIC chips. The dither frequencies may, forexample, be in the range of low frequencies of about 10 KHz to about 100KHz, although this range can extend on either side of this specificrange as exemplified in the embodiment of FIG. 37where the range of tonefrequencies is from 1 KHz to 10 KHz. These channel tags are also highlyuseful in allowing monitoring of any channel in the transmitter network,particularly at the optical receiver side, with a single tap,photodetector, and accompanying low-speed electronic circuitry to detectand monitor incoming individual channels signals.

Thus, in partial summary of the embodiment shown in FIG. 37, an externaltap coupler 320 at the output of TxPIC 10 couples a small portion of themultiplexed signal to external photodetector 351. An integratedphotodetector on chip 10, such as either PD 235A or 235B, may also beused for the same purpose. Each DFB laser 12 has its driver currentmodulated by a dither current, a low frequency AC component having amodulation depth 376 and frequency at 374. The AC modulation currentcauses a corresponding low frequency variation in laser wavelength whichis sufficiently small in intensity as to not affect the detectionquality of photodiode arrays in Receiver RxPIC chips. Electronicfrequency filters 358 permit the response at each dither frequency to bemeasured from the photodetector response. Feedback electronic circuitry360 also provides a control loop for adjusting the dither modulationdepth 376 and bias point of the frequency dither. Since each laser 12has its own unique dither frequency, its wavelength and power responsemay be identified by using a lock-in technique to analyze the frequencyresponse of the photodetector at the dither frequency.

A controller may monitor the change in power output at the ditherfrequency and employ a control loop to approach an operating pointcentered on the peak of the AWG passband. It should also be understoodthat dithering for purposes of monitoring can be performed relative toonly one laser on the TxPIC 10 while the wavelength of the other on-chiplasers are initially locked to a standardized grid wavelength. In thecase here, it is preferred that all of the laser sources have beencharacterized to substantially have the same wavelength shift responseso that any determined wavelength change for the one monitoring lasermay also be may made to the other on-chip lasers. Alternatively, morethan one laser with different tone frequencies may be used for thispurpose. Thus, every laser may be dithered and independently locked orjust a few lasers, like two or more lasers, may be dithered and locked,or only one laser, sequentially one at a time on the TxPIC, is ditheredand wavelength locked. In this latter mentioned alternative, one channelmay be locked, and the other channels adjusted based on the offset intemperature/current required to lock the one laser. Alternatively, thelocking may be cycled sequentially among lasers. If the array locking iscycled, an interpolation method may be used for some of the channels. Itshould be understood that in all of the foregoing cases, where one ormore or all of lasers are locked to the peak of the AWG passbandresponse, it should be understood that the laser wavelength may, aswell, be locked to either side edge of the passband response rather thanthe peak.

Other embodiments for detection of a small portion of the AWGmultiplexed signal output include an integrated optical detector on chip10 for detecting the dithered output of AWG 50 using an integratedwaveguide tap or other on-chip coupling means. Alternatively, a detectoror photodiode may be directly coupled to the second slab waveguideregion to receive a 2^(ND) order output signal directly from output slab54. In general, AWG 50 is designed to couple multiplexed signal channelsinto its 0^(th) order Brillouin zone. Some power is always coupled tohigher order Brillouin zones, e.g., 1^(st) and 2^(nd) order Brillouinzones. The light focused in slab 54 on the higher order Brillouin zonesis a replica of the 0^(th) order cone. As an illustrative example for anAWG with an output star coupler loss of approximately 1 dB, the totalpower in the 1^(st) Brillouin zone is approximately 10 dB lower than thepower in the 0^(th) Brillouin zone. The power coupled to higher orderBrillouin zones may be tapped for on-chip optical detection. Anintegrated optical detector, e.g., a PIN photodiode, may be located atthe focal point of a higher order Brillouin zone as previouslyindicated. Alternatively, a waveguide may be placed at the focal pointof a higher order Brillouin zone to couple the higher order Brillouinzone power to an optical detector, such as waveguide 234A or 234B andphotodetector 235A or 235B.

The advantages of wavelength locking system 350 in FIG. 37 are: (1)Wavelengths can be locked in a low cost manner using a minimum ofadditional components (a 1% tap, photodetector, and some very low speedelectronic circuitry) due to the deployment of an already existing onchip AWG 50 providing for filtered frequency differentiation, (2) Thelaser source wavelength grid is automatically aligned substantially tothe AWG wavelength grid, (3) The same setup can be employed for anyarbitrary channel spacing which is set by the AWG parameters and (4) Theuse of unique identifying tags for each channel can be utilized forother purposes such as per-channel power diagnostics at substantially noadded cost.

Alternatively relative to the embodiment shown in FIG. 37, AWG 50 may bedesigned to also include an additional channel and the TxPIC may befabricated to include an extra on-chip laser source employed forwavelength locking all of the laser sources relative to the wavelengthgrid of AWG 50. A TxPIC may have a first order Brillouin zone, an extraset of waveguides in the AWG where the light is tapped directly off atthe second free space region or slab via a integrated detector or isprovided with a passive waveguide from each extra waveguide output fromthe second free space region to a PD integrated on the TxPIC. In eithercase, a pair of on-chip photodetectors, such as PDs 235A and 235B inFIG. 37, is arranged with a respective photodetector positioned onadjacent sides of the passband center of a particular wavelength beingmonitored or the passband wavelength center of the AWG itself. In eithercase, the amount of wavelength offset from the wavelength grid of theAWG can be measured and utilized to re-center the laser wavelength gridto the AWG grid. In the particular embodiment of FIG. 37, PINphotodiodes 235A and 235B are fabricated in the higher order ± Brillouinzones, e.g., the −1 and +1 Brillouin zones 234A and 234B, of AWG 50. Thetwo photodiodes 235 are disposed to detect on opposite sides of the AWGpassband. Each DFB laser may be dithered at the same frequency or adifferent frequency. A DFB laser 12 is aligned to the AWG passband whenits wavelength is tuned such that the two photodiodes 235A and 235B havea balanced AC output, i.e., outputs of the same magnitude. Moregenerally, a balanced ratio between these AC photodiodes can be deployedas a setpoint for a reference. For the purposes of making this passbandtest for each DFB laser 12 on TxPIC chip 10, the DFB lasers may be eachdithered sequentially, one at the time, at the same tone frequency or atdifferent tone frequencies, i.e., all at once.

Additional output waveguides and/or detectors may be placed off-centerat the output edge of the slab waveguide region to receive light, forexample, from a dummy channel formed on the TxPIC chip. Twophotodetectors may be arranged adjacent to the passband center of thedummy channel wavelength. In this approach, a dummy laser, comprisingthe dummy channel, is coupled as an input to first slab 52 of AWG 54.AWG 54 may include two dummy channel output waveguides and correspondingdual photodiodes positioned to receive light at wavelengths, forexample, λ_(d)+Δλ, and λ_(d)−Δλ, where λ_(d) is a dummy channel targetwavelength and Δλ is a wavelength offset from the target dummywavelength. When the dummy channel wavelength is tuned to its targetwavelength, both optical detectors will have a desired ratio of powerlevels. The dummy laser may be tuned in wavelength until the power ratiois correctly set in the two spatially disposed photodiodes at λ₀+Δλ andλ₀−Δλ, where λ₀ is center wavelength. When the power ratio is correctlyset, the center wavelength λ₀ is aligned to, for example, the passbandcenter frequency of the AWG. The detector scheme of employing twodiscrete, spatial photodiodes is known in the art but the use, asexplained herein, in connection with TxPIC chips disclosed in thisapplication has not been previously disclosed as far as the applicantsknow.

In all of the foregoing AWG dithering embodiments, a single on-chiplaser out of a plurality of such on-chip laser sources may include adither tone for the purpose of wavelength locking of all of the otherlaser sources.

The passband response of the AWG will depend upon the refractive indexof the AWG. For example, the refractive index of each AWG may beadjusted by temperature tuning, as previously explained. The passbandresponse of the AWG may be characterized in the factory to set anoperating temperature of the AWG for which the passband response of theAWG is aligned to the ITU wavelength channel grid, i.e., the peaktransmissivity of the AWG is approximately aligned with the desiredwavelength channels to achieve acceptable insertion loss level.

The output of TxPIC 10 may include an inline optical amplifier 326 toboost the multiplexed signal launched onto optical fiber link 328.Amplifier 326 may, for example, be an EDFA. The output of TxPIC chip 10may also include variable optical attenuator (VOA) 327 to attenuate orotherwise extinguish any output signal on optical link 328 during thestartup and calibration phase of chip 10 and feedback system 350 until asteady, stabilized operating state is reached. This calibration phaseincludes the checking and tuning of the individual wavelengths of DFBlasers 12 on chip 12 for their optimized operating wavelengthssubstantially matching the wavelength grid of AWG 50. When thecalibration phase is complete, VOA 327 is turned off to permit thenormal transmission of multiplexed channel signals on optical link 328.In this way, an optical receiver will not received calibration dataconfusing to the operation of such an optical receiver. It should becarefully understood that VOA 327 is not the only component to performsuch a shut-off function, as there other optical components that couldalso perform this function, such as an optical switch or a Mach-Zehnderinterferometer, to switch out any optical power during the calibrationphase.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. Thus, the invention describedherein is intended to embrace all such alternatives, modifications,applications and variations as may fall within the spirit and scope ofthe appended claims.

1. A method of in-wafer testing of monolithic photonic integratedcircuits (PICs) formed in a semiconductor wafer where each such circuitcomprises two or more integrated electro-optic components, one of eachin tandem forming a signal channel in the circuit, and comprising thesteps of: providing a first integrated photodetector at a rear end ofeach signal channel and a second integrated photodetector at forward endof each signal channel; sequentially operating a first of a selectedchannel electro-optic component in a selected circuit to monitor lightoutput from a channel via its first corresponding channel photodetectorand adjusting its operating characteristics by detecting that channelelectro-optic component output via its second corresponding channelphotodetector to provide first calibration data; sequentially operatinga second of a selected channel electro-optic component in the selectedcircuit to monitor signal output from the second selected channelelectro-optic component via its second corresponding channelphotodetector and adjusting its operating characteristics by detectingthat channel electro-optic component output via its second correspondingchannel photodetector to provide second calibration data; and thereafterstoring the first and second calibration data for each circuit channelfor the selected circuit for future reference.
 2. The method of claim 1wherein the first electro-optic component operating characteristics areits applied bias or its operational wavelength.
 3. The method of claim 1wherein the second electro-optic component operating characteristics areits applied bias, extinction ratio or chirp.
 4. The method of claim 1comprising the further step of fabricating the in-wafer circuit to be anoptical transmitter photonic integrated circuit (TXPIC) with a pluralityof signal channels each comprising an in-tandem laser source andmodulator.
 5. The method of claim 4 comprising the further step ofcombining outputs in the circuit from the signal channels to provide amultiplexed signal on an output of the circuit.
 6. The method of claim 4comprising the further step of employing the first calibration data fortuning the laser source wavelength to a predetermined wavelength.
 7. Themethod of claim 6 wherein the step of tuning is accomplished byadjusting a bias current to the laser source in each signal channel. 8.The method of claim 6 wherein the step of tuning is accomplished byadjusting heat applied to a local heater for the laser source in eachsignal channel.
 9. The method of claim 4 comprising the further step offabricating in each signal channel a semiconductor optical amplifier(SOA) and adjusting the applied bias current to each SOA to provide adesired output power from each laser.
 10. The method of claim 4comprising the further step of employing the second calibration data foradjusting the bias point of operation of the modulator to achieveoptimum extinsion ratio and chirp.
 11. A method of in-wafer testing ofmonolithic photonic integrated circuits (PICs) formed in a semiconductorwafer, each circuit comprising an array of integrated laser sources andan array of integrated electro-optic modulators, one each for each ofthe laser sources, and an optical combiner to receive the modulatedsignal outputs from each of the electro-optic modulators comprising thesteps of: providing a first integrated photodetector at a rear facet ofeach of said laser sources in each circuit; providing a secondintegrated photodetector between each of said modulators and saidoptical combiner in each circuit; sequentially operating each of thelaser sources in a selected circuit to monitor light output from a lasersource via its first corresponding photodetector and calibrate itsoperating characteristics by detecting the laser source output via itssecond corresponding photodetector thereby providing first calibrationdata; sequentially operating each of the modulators in the selectedcircuit to monitor signal output from a modulator via its secondcorresponding photodetector and adjusting its bias point of operation toachieve optimum extinsion ratio and chirp thereby providing secondcalibration data; and thereafter storing the first and secondcalibration data for each circuit for future reference.
 12. The methodof claim 11 further comprising the steps of: cleaving each circuit fromthe wafer forming a photonic integrated circuit (PIC) chip; and cleavingfrom each chip the first integrated photodetectors.
 13. The method claim11 further comprising the steps of: cleaving each circuit from the waferforming a photonic integrated circuit (PIC) chip; and operating eachsecond photodetector either as a monitoring photodiode of the signallight from each modulator.
 14. The method of claim 11 further comprisingthe steps of: cleaving each circuit from the wafer forming a photonicintegrated circuit (PIC) chip; and biasing each second photodetector tobe transparent to light generated from its corresponding laser source.15. The method claim 14 wherein the biasing is a small positive value orzero.
 16. The method of claim 11 wherein said first and secondphotodetectors are avalanche photodiodes (APDs) or PIN photodiodes. 17.A method of in-wafer testing of monolithic photonic integrated circuit(PICs) formed in a semiconductor wafer, each circuit comprising an arrayof integrated laser sources and an array of integrated electro-opticmodulators, one each for each of the laser sources, and an opticalcombiner to receive the modulated signal outputs from each of theelectro-optic modulators comprising the steps of: providing anintegrated photodetector between each of said laser sources and saidmodulators or between said modulators and said optical combiner in eachcircuit; sequentially operating each of the laser sources in a selectedchip to monitor light output from a laser source via its correspondingphotodetector and calibrate its operating characteristics by detectingthe laser source output via its corresponding photodetectors;sequentially operating each of the modulators in the selected chip tomonitor signal output from a modulator via its correspondingphotodetector and adjusting its bias point of operation to achieveoptimum extinsion ratio and chirp thereby providing calibration datarelating to the circuit; and thereafter storing the calibration data foreach circuit for future reference.
 18. The method of claim 1.7 furthercomprising the steps of: determining that one or more of the lasersources or modulators in particular circuit in the wafer do not functionproperly; cleaving the circuits from the wafer forming a photonicintegrated circuit (PIC) chips; and thereafter discarding the chips thatdo not function properly.
 19. The method of claim 17 further comprisingthe steps of: determining that one or more of the laser sources ormodulators in a substantial number of circuits in the wafer do notfunction properly; and thereafter discarding the wafer thereby savingresources normally consumed where the circuits are tested after beingcleaved from the wafer.